note to myself and anyone with same problem who finds the thread:

stumbled on psync: A naive tool for syncing partitions using rsync which sounds like it’s at lest trying to do this task.

No documentation, no users, no project changes in 6 years… Will give it a try another time.

I recommend just backing up the files.

But if you really have to back up the disk image, dd a copy of it, mount the copied image as loopback device, write the loopback filesystem full of /dev/zero, sync, delete the zero file(s), unmount, cp --sparse=always and store the result.

The reason for using the loopback image step is to prevent wearing out the SD card with writing the free space full of zeros every time you make a backup.

There may be an existing tool for this, but I don’t remember it.

You’re looking for a block level incremental backup solution. This can either be achieved using filesystem based snapshots (ZFS, BRRFS) or using dedicated programs. I know rdiff-backup , restic and duplicity use block-level diffs, not sure about rsnapshot.

I haven’t tried such a thing, but I remember ZFS has an option for block deduplication.

So you would set up a ZFS with block deduplication (and probably without compression - try this point out), and then you make your backup images with the dd tool and the correct block size.

Now you make always full copies and have them as normal files but they take only the disk space of the differences.

let me know if I am going about the problem in a wrong way.

I would not say “wrong way”. I’s fun to think about such things and try them out.

On the other hand I think a FAT32 can have only 32Gb. I would not mind having many of them lying around on my home NAS that has 12 Tb on RAID :-)