I fought these messages yesterday. In my case the reason is that topDir 
- the directory where the backup data is stored - is a separate 
file-system, and that BackupPC has problems dealing with that situation. 
Googling provided 2 alternative solutions (there appears to be quite an 
epidemy of people suffering from this problem):

1. Make /var/lib/backuppc an empty directory (if currently you have data 
there, start by saving it elsewhere), and than mount your topDir 
file-system at /var/lib/backuppc (also documented in 
http://www.cs.umd.edu/~cdunne/projs/backuppc_guide.html).

2. Use the --bind option of mount (the Google reference suggests -o 
bind, I guess another and probably obsolete flavour of --bind, not 
documented on my Mandriva platform). I start by mounting my file system 
for backup data quite traditionally at the mount point "/backup", but 
after that I add an additional mount instruction:

mount --bind /backup /var/lib/backuppc

That acts like a hard link between /backup and /var/lib/backuppc - you 
see the same data through either mount point (and, evidently, you must 
again start by saving any data at /var/lib/backuppc) - and BackupPC 
appears to be happy, no more error messages.

You will probably want to avoid the explicit mount --bind ... 
instruction: this is done by adding to your /etc/fstab - I placed it 
directly after the "mount /dev/xxx /backup ..." instruction - the 
following line:

/backup /var/lib/backuppc none rw,bind 0 0

with the result that the 2 mount points get mounted at boot time. I also 
changed owner and group of /backup to the backup user - but there I am 
less sure how correct or imperative that is.

I read that a new release (3.1.1) of backuppc is being prepared - short 
of fixing this problem, it might be a good idea to add a couple of words 
to the documentation, warning about this restriction in the 
configurability of the topDir variable.

Good luck!


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