[rdiff-backup-users] undo backup operation

[talarek]

Hello, I have situation like this: 
Soruce folder (300 GB, Windows share mounted by cifs)
Backup folder (350 GB, backup + some increments, 500 GB disk quota)

I'm doing "rdiff-backup source backup" and everything is ok but unfortunetly I 
didn't do protection in my backup script in case of bad mount cifs source 
folder (source folder is empty after lost connection to the source server). 

Last night I've lost connection to server, rdiff-backup backed up this folder 
and now I have backup folder only containing rdiff-backup-data. 

I can do "rdiff-backup source backup" again but:
1) all files from source will be copied to backup folder (this will take about 
1 week :)
2) backup folder will be 300 GB + 350 GB = 750 GB  but I only have 500 GB (I 
don't want to waste disk space on backup server)

Is there any way to undo last (or more) backup operation to avoid described 
problem? 

Best regards