The folder /backup is the root of the disk. I mounted the disk there, doing the ls -l /backup showed all the root folders on the disk. Perhaps there is something going on with the PC folders, as the lost+found and trash folders are both empty.

I'm not sure how I can go about determining if a particular backup is using the pool or just storing the files in the PC folder. What's the best way to check if a given backup set is represented in the pool or not? Would knowing the size of all the pc folders help narrow it down?

I'm not sure if this is the best way to check the hard linking, but here's a test I thought might be helpful. I did this command to see if a common file in these backups are pointing to the same inodes.

On Tue, Oct 29, 2013 at 6:07 PM, <backuppc AT kosowsky DOT org> wrote:

Les Mikesell wrote at about 16:51:12 -0500 on Tuesday, October 29, 2013:

 > On Tue, Oct 29, 2013 at 4:30 PM, Timothy J Massey <tmassey AT obscorp DOT com> wrote:
 > >
 > >
 > > Check lost+found and trash while you're at it and see what's in there.  They should both be empty.
 > >
 > > I'm with Jeff:  I think that you have multiple PC trees that are not part of the pool.  How you managed that I'm not sure.  But you need to find those files and clean them up.  Start with Jeff's command and go from there.
 >
 > This could happen if the backups were originally on a different
 > filesystem and were copied over without preserving the pool hardlinks.
 >  For example if you rsync an individual pc directory into place,
 > subsequent rsync runs will link against those copies for existing
 > files but will only make the pool links for new/changed files.
 >
 > --

It also can happen if you have filesystems with flaky hard linking --
I once had that issue with a bad user-space nfs module.