[rdiff-backup-users] Backing up the same unchanged SD card in two distributions processes all files as changed

[白い熊＠相撲道]

User case:

I have an SD card that I back up to an external harddisk. On the same 
machine, however use a number of GNU/Linux distributions.

If I back it up from the same distribution repeatedly, without any 
change to the SD file system, no file is processed as changed - that's 
the way it should be.

If I reboot into another distribution, and run the back up there, it 
will mark all the files as changed, and copy all the data to the backup 
directory - clearly wrong.

Why is this? I'm not sure what's causing this - seems to be distribution 
independent, simply boot into some other distribution and its 
rdiff-backup thinks all the files have been modified. Why?

How can I have it not do this and process the files as unchanged?
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白い熊＠相撲道