diff --git a/internal/pkg/onuadaptercore/file_download_manager.go b/internal/pkg/onuadaptercore/file_download_manager.go
index 4793e69..4948323 100644
--- a/internal/pkg/onuadaptercore/file_download_manager.go
+++ b/internal/pkg/onuadaptercore/file_download_manager.go
@@ -43,14 +43,14 @@
 	cFileStateDlSucceeded
 	cFileStateDlFailed
 	cFileStateDlAborted
-	cFileStateDlInvalid
 )
 
 type downloadImageParams struct {
 	downloadImageName       string
+	downloadImageURL        string
 	downloadImageState      fileState
 	downloadImageLen        int64
-	downloadImageCrc        uint32
+	downloadImageCRC        uint32
 	downloadActive          bool
 	downloadContextCancelFn context.CancelFunc
 }
@@ -59,6 +59,7 @@
 
 //fileDownloadManager structure holds information needed for downloading to and storing images within the adapter
 type fileDownloadManager struct {
+	mutexFileState        sync.RWMutex
 	mutexDownloadImageDsc sync.RWMutex
 	downloadImageDscSlice []downloadImageParams
 	dnldImgReadyWaiting   map[string]requesterChannelMap
@@ -91,38 +92,45 @@
 	return dm.dlToAdapterTimeout
 }
 
-//ImageExists returns true if the requested image already exists within the adapter
-func (dm *fileDownloadManager) ImageExists(ctx context.Context, aImageName string) bool {
-	logger.Debugw(ctx, "checking on existence of the image", log.Fields{"image-name": aImageName})
-	dm.mutexDownloadImageDsc.RLock()
-	defer dm.mutexDownloadImageDsc.RUnlock()
-
-	for _, dnldImgDsc := range dm.downloadImageDscSlice {
-		if dnldImgDsc.downloadImageName == aImageName {
-			//image found (by name)
-			return true
-		}
-	}
-	//image not found (by name)
-	return false
-}
-
-//StartDownload returns true if the download of the requested image could be started for the given file name and URL
-func (dm *fileDownloadManager) StartDownload(ctx context.Context, aImageName string, aURLCommand string) error {
+//StartDownload returns FileState and error code from download request for the given file name and URL
+func (dm *fileDownloadManager) StartDownload(ctx context.Context, aImageName string, aURLCommand string) (fileState, error) {
 	logger.Infow(ctx, "image download-to-adapter requested", log.Fields{
 		"image-name": aImageName, "url-command": aURLCommand})
-	loDownloadImageParams := downloadImageParams{
-		downloadImageName: aImageName, downloadImageState: cFileStateDlStarted,
-		downloadImageLen: 0, downloadImageCrc: 0}
-	//try to download from http
-	var err error
-	if err = dm.downloadFile(ctx, aURLCommand, cDefaultLocalDir, aImageName); err == nil {
-		dm.mutexDownloadImageDsc.Lock()
+	// keep a semaphore over the complete method in order to avoid parallel entrance to this method
+	// otherwise a temporary file state 'Started' could be indicated allowing start of ONU upgrade handling
+	// even though the download-start to adapter may fail (e.g on wrong URL) (delivering inconsistent download results)
+	// so once called the download-start of the first call must have been completely checked before another execution
+	// could still be limited to the same imageName, but that should be no real gain
+	dm.mutexFileState.Lock()
+	defer dm.mutexFileState.Unlock()
+	dm.mutexDownloadImageDsc.Lock()
+	var fileState fileState
+	var exists bool
+	if fileState, exists = dm.imageExists(ctx, aImageName, aURLCommand); !exists {
+		loDownloadImageParams := downloadImageParams{
+			downloadImageName: aImageName, downloadImageURL: aURLCommand, downloadImageState: cFileStateDlStarted,
+			downloadImageLen: 0, downloadImageCRC: 0}
 		dm.downloadImageDscSlice = append(dm.downloadImageDscSlice, loDownloadImageParams)
 		dm.mutexDownloadImageDsc.Unlock()
+		//start downloading from server
+		var err error
+		if err = dm.downloadFile(ctx, aURLCommand, cDefaultLocalDir, aImageName); err == nil {
+			//indicate download started correctly, complete download may run in background
+			return cFileStateDlStarted, nil
+		}
+		//return the error result of the download-request
+		return cFileStateUnknown, err
 	}
-	//return the result of the start-request to comfort the core processing even though the complete download may go on in background
-	return err
+	dm.mutexDownloadImageDsc.Unlock()
+	if fileState == cFileStateUnknown {
+		//cannot simply remove the existing file here - might still be used for running upgrades on different devices!
+		//  (has to be removed by operator - cancel API)
+		logger.Errorw(ctx, "image download requested for existing file with different URL",
+			log.Fields{"image-description": aImageName, "url": aURLCommand})
+		return fileState, fmt.Errorf("existing file is based on different URL, requested URL: %s", aURLCommand)
+	}
+	logger.Debugw(ctx, "image download already started or done", log.Fields{"image-description": aImageName})
+	return fileState, nil
 }
 
 //GetImageBufferLen returns the length of the specified file in bytes (file size) - as detected after download
@@ -139,6 +147,8 @@
 }
 
 //GetDownloadImageBuffer returns the content of the requested file as byte slice
+//precondition: it is assumed that a check is done immediately before if the file was downloaded to adapter correctly from caller
+//  straightforward approach is here to e.g. immediately call and verify GetImageBufferLen() before this
 func (dm *fileDownloadManager) GetDownloadImageBuffer(ctx context.Context, aFileName string) ([]byte, error) {
 	file, err := os.Open(filepath.Clean(cDefaultLocalDir + "/" + aFileName))
 	if err != nil {
@@ -221,6 +231,24 @@
 
 // FileDownloadManager private (unexported) methods -- start
 
+//imageExists returns current ImageState and if the requested image already exists within the adapter
+//precondition: at calling this method mutexDownloadImageDsc must already be at least RLocked by the caller
+func (dm *fileDownloadManager) imageExists(ctx context.Context, aImageName string, aURL string) (fileState, bool) {
+	logger.Debugw(ctx, "checking on existence of the image", log.Fields{"image-name": aImageName})
+	for _, dnldImgDsc := range dm.downloadImageDscSlice {
+		if dnldImgDsc.downloadImageName == aImageName {
+			//image found (by name)
+			if dnldImgDsc.downloadImageURL == aURL {
+				//image found (by name and URL)
+				return dnldImgDsc.downloadImageState, true
+			}
+			return cFileStateUnknown, true //use fileState to indicate URL mismatch for existing file
+		}
+	}
+	//image not found (by name)
+	return cFileStateUnknown, false
+}
+
 //imageLocallyDownloaded returns true if the requested image already exists within the adapter
 //  requires mutexDownloadImageDsc to be locked (at least RLocked)
 func (dm *fileDownloadManager) imageLocallyDownloaded(ctx context.Context, aImageName string) bool {
@@ -294,6 +322,7 @@
 	urlBase, err1 := url.Parse(aURLCommand)
 	if err1 != nil {
 		logger.Errorw(ctx, "could not set base url command", log.Fields{"url": aURLCommand, "error": err1})
+		dm.removeImage(ctx, aFileName, false) //wo FileSystem access
 		return fmt.Errorf("could not set base url command: %s, error: %s", aURLCommand, err1)
 	}
 	urlParams := url.Values{}
@@ -303,6 +332,7 @@
 	reqExist, errExist2 := http.NewRequest("HEAD", urlBase.String(), nil)
 	if errExist2 != nil {
 		logger.Errorw(ctx, "could not generate http head request", log.Fields{"url": urlBase.String(), "error": errExist2})
+		dm.removeImage(ctx, aFileName, false) //wo FileSystem access
 		return fmt.Errorf("could not  generate http head request: %s, error: %s", aURLCommand, errExist2)
 	}
 	ctxExist, cancelExist := context.WithDeadline(ctx, time.Now().Add(3*time.Second)) //waiting for some fast answer
@@ -313,6 +343,7 @@
 		if respExist == nil {
 			logger.Errorw(ctx, "http head from url error - no status, aborting", log.Fields{"url": urlBase.String(),
 				"error": errExist3})
+			dm.removeImage(ctx, aFileName, false) //wo FileSystem access
 			return fmt.Errorf("http head from url error - no status, aborting: %s, error: %s",
 				aURLCommand, errExist3)
 		}
@@ -322,6 +353,7 @@
 		if respExist.StatusCode != http.StatusMethodNotAllowed {
 			logger.Errorw(ctx, "http head from url: file does not exist here, aborting", log.Fields{"url": urlBase.String(),
 				"error": errExist3, "status": respExist.StatusCode})
+			dm.removeImage(ctx, aFileName, false) //wo FileSystem access
 			return fmt.Errorf("http head from url: file does not exist here, aborting: %s, error: %s, status: %d",
 				aURLCommand, errExist3, respExist.StatusCode)
 		}
@@ -403,7 +435,7 @@
 
 //removeImage deletes the given image according to the Image name from filesystem and downloadImageDscSlice
 func (dm *fileDownloadManager) removeImage(ctx context.Context, aImageName string, aDelFs bool) {
-	logger.Debugw(ctx, "remove the image from Adapter", log.Fields{"image-name": aImageName})
+	logger.Debugw(ctx, "remove the image from Adapter", log.Fields{"image-name": aImageName, "del-fs": aDelFs})
 	dm.mutexDownloadImageDsc.RLock()
 	defer dm.mutexDownloadImageDsc.RUnlock()
 
diff --git a/internal/pkg/onuadaptercore/omci_onu_upgrade.go b/internal/pkg/onuadaptercore/omci_onu_upgrade.go
index 49b320d..657c857 100644
--- a/internal/pkg/onuadaptercore/omci_onu_upgrade.go
+++ b/internal/pkg/onuadaptercore/omci_onu_upgrade.go
@@ -624,7 +624,7 @@
 	} else {
 		fileLen, err = oFsm.pDownloadManager.getImageBufferLen(ctx, oFsm.pImageDsc.Name, oFsm.pImageDsc.LocalDir)
 	}
-	if err != nil || fileLen > int64(cMaxUint32) {
+	if err != nil || fileLen == 0 || fileLen > int64(cMaxUint32) {
 		oFsm.volthaDownloadReason = voltha.ImageState_UNKNOWN_ERROR //something like 'LOCAL_FILE_ERROR' would be better (proto)
 		oFsm.mutexUpgradeParams.Unlock()
 		logger.Errorw(ctx, "OnuUpgradeFsm abort: problems getting image buffer length", log.Fields{
@@ -638,11 +638,11 @@
 	}
 
 	//copy file content to buffer
-	oFsm.imageBuffer = make([]byte, fileLen)
+	var imageBuffer []byte
 	if oFsm.useAPIVersion43 {
-		oFsm.imageBuffer, err = oFsm.pFileManager.GetDownloadImageBuffer(ctx, oFsm.imageIdentifier)
+		imageBuffer, err = oFsm.pFileManager.GetDownloadImageBuffer(ctx, oFsm.imageIdentifier)
 	} else {
-		oFsm.imageBuffer, err = oFsm.pDownloadManager.getDownloadImageBuffer(ctx, oFsm.pImageDsc.Name, oFsm.pImageDsc.LocalDir)
+		imageBuffer, err = oFsm.pDownloadManager.getDownloadImageBuffer(ctx, oFsm.pImageDsc.Name, oFsm.pImageDsc.LocalDir)
 	}
 	if err != nil {
 		oFsm.volthaDownloadReason = voltha.ImageState_UNKNOWN_ERROR //something like 'LOCAL_FILE_ERROR' would be better (proto)
@@ -656,12 +656,17 @@
 		}(pBaseFsm)
 		return
 	}
+	//provide slice capacity already with the reserve of one section to avoid inflation of the slice to double size at append
+	oFsm.imageBuffer = make([]byte, fileLen, fileLen+cOmciDownloadSectionSize)
+	//better use a copy of the read image buffer in case the buffer/file is modified from outside,
+	//  this also limits the slice len to the expected maximum fileLen
+	copy(oFsm.imageBuffer, imageBuffer)
 
 	oFsm.noOfSections = uint32(fileLen / cOmciDownloadSectionSize)
 	if fileLen%cOmciDownloadSectionSize > 0 {
 		bufferPadding := make([]byte, cOmciDownloadSectionSize-uint32((fileLen)%cOmciDownloadSectionSize))
 		//expand the imageBuffer to exactly fit multiples of cOmciDownloadSectionSize with padding
-		oFsm.imageBuffer = append(oFsm.imageBuffer[:(fileLen)], bufferPadding...)
+		oFsm.imageBuffer = append(oFsm.imageBuffer, bufferPadding...)
 		oFsm.noOfSections++
 	}
 	oFsm.origImageLength = uint32(fileLen)
diff --git a/internal/pkg/onuadaptercore/openonu.go b/internal/pkg/onuadaptercore/openonu.go
index ef74c68..61f3c03 100644
--- a/internal/pkg/onuadaptercore/openonu.go
+++ b/internal/pkg/onuadaptercore/openonu.go
@@ -633,7 +633,7 @@
 	if request != nil && len((*request).DeviceId) > 0 && (*request).Image.Version != "" {
 		loResponse := voltha.DeviceImageResponse{}
 		imageIdentifier := (*request).Image.Version
-		downloadedToAdapter := false
+		downloadStartDone := false
 		firstDevice := true
 		var vendorID string
 		var onuVolthaDevice *voltha.Device
@@ -666,28 +666,24 @@
 				if firstDevice {
 					//start/verify download of the image to the adapter based on first found device only
 					//  use the OnuVendor identification from first given device
+					//  note: if the request was done for a list of devices on the Voltha interface, rwCore
+					//  translates that into a new rpc for each device, hence each device will be the first device in parallel requests!
 					firstDevice = false
 					vendorID = onuVolthaDevice.VendorId
 					imageIdentifier = vendorID + imageIdentifier //head on vendor ID of the ONU
-					logger.Debugw(ctx, "download request for file", log.Fields{"image-id": imageIdentifier})
-
-					if !oo.pFileManager.ImageExists(ctx, imageIdentifier) {
-						logger.Debugw(ctx, "start image download", log.Fields{"image-description": request})
-						// Download_image is not supposed to be blocking, anyway let's call the DownloadManager still synchronously to detect 'fast' problems
-						// the download itself is later done in background
-						if err := oo.pFileManager.StartDownload(ctx, imageIdentifier, (*request).Image.Url); err == nil {
-							downloadedToAdapter = true
-						}
-						//else: treat any error here as 'INVALID_URL' (even though it might as well be some issue on local FS, eg. 'INSUFFICIENT_SPACE')
-						// otherwise a more sophisticated error evaluation is needed
-					} else {
-						// image already exists
-						downloadedToAdapter = true
-						logger.Debugw(ctx, "image already downloaded", log.Fields{"image-description": imageIdentifier})
+					logger.Infow(ctx, "download request for file",
+						log.Fields{"device-id": loDeviceID, "image-id": imageIdentifier})
+					// call the StartDownload synchronously to detect 'immediate' download problems
+					// the real download itself is later done in background
+					if fileState, err := oo.pFileManager.StartDownload(ctx, imageIdentifier, (*request).Image.Url); err == nil {
 						// note: If the image (with vendorId+name) has already been downloaded before from some other
-						//   valid URL, the current URL is just ignored. If the operators want to ensure that the new URL
+						//   valid URL, the current download request is not executed (current code delivers URL error).
+						//   If the operators want to ensure that the new URL
 						//   is really used, then they first have to use the 'abort' API to remove the existing image!
 						//   (abort API can be used also after some successful download to just remove the image from adapter)
+						if fileState == cFileStateDlSucceeded || fileState == cFileStateDlStarted {
+							downloadStartDone = true
+						} //else fileState may also indicate error situation, where the requested image is not ready to be used for other devices
 					}
 				} else {
 					//for all following devices verify the matching vendorID
@@ -697,11 +693,11 @@
 						vendorIDMatch = false
 					}
 				}
-				if downloadedToAdapter && vendorIDMatch {
+				if downloadStartDone && vendorIDMatch {
 					// start the ONU download activity for each possible device
-					logger.Debugw(ctx, "image download on omci requested", log.Fields{
+					logger.Infow(ctx, "request image download to ONU on omci ", log.Fields{
 						"image-id": imageIdentifier, "device-id": loDeviceID})
-					//onu upgrade handling called in background without immediate error evaluation here
+					// onu upgrade handling called in background without immediate error evaluation here
 					//  as the processing can be done for multiple ONU's and an error on one ONU should not stop processing for others
 					//  state/progress/success of the request has to be verified using the Get_onu_image_status() API
 					go handler.onuSwUpgradeAfterDownload(ctx, request, oo.pFileManager, imageIdentifier)
@@ -710,7 +706,9 @@
 					loDeviceImageState.ImageState.ImageState = voltha.ImageState_IMAGE_UNKNOWN
 				} else {
 					loDeviceImageState.ImageState.DownloadState = voltha.ImageState_DOWNLOAD_FAILED
-					if !downloadedToAdapter {
+					if !downloadStartDone {
+						// based on above fileState more descriptive error codes would be possible, e.g
+						//   IMAGE_EXISTS_WITH_DIFFERENT_URL - would require proto buf update
 						loDeviceImageState.ImageState.Reason = voltha.ImageState_INVALID_URL
 					} else { //only logical option is !vendorIDMatch
 						loDeviceImageState.ImageState.Reason = voltha.ImageState_VENDOR_DEVICE_MISMATCH