According to the manual:
If the backup is incremental the client calculates a hash of 512 kbyte chunks and compares it to the previous image backup. If the hash of the chunk has not changed it does not transfer this chunk to the server, otherwise it does. Per default the server writes the image data directly into a VHD file. If enabled in the advanced configuration the server writes the image data to a temporary file first. The temporary files have a maximum size of 1GB. After this size is exceeded the server continues with a new temporary file. The image data is written to a VHD file in parallel and is located in the client directory in the backup storage location. The VHD file’s name is ’Image__<YYMMDD_HHMM>.vhd’.being the drive letter of the backed up partition and YY the current year, MM the current month, DD the current day in the month and HHMM the hour and minute the image backup was started.
I’ve seen topics posted that image incremental image backups take as long as full image backups, this does not seem to be true according to the above snippet from the manual, am I reading this incorrectly or does calculating and comparing hashes take a long time. This backup is nearly 5TB for a full, so this would be great information if anyone can answer.