Comment 9 for bug 445154

It might be appropriate here to also note that backuppc now *requires* hardlinks from it's
archive to /var/lib/backuppc/cpool in order for pooling to work (the main reason for using
backuppc). Previous versions of backuppc did not check if the creation of these hardlinks
was successful, thus people could create per-machine symlinks in /var/lib/backuppc/pc
which pointed to another filesystem, and it would "just work" (at the expense of those not
pooling, thus wasting a LOT of disk space). Recent versions do check to make sure
pooling is working, and if symlinks have been used as described above, the machines
symlinked to other filesystems will not backup, and start throwing the same error message
as in this bug.

You can use an external disk and/or different filesystem, but everything needs to live
under /var/lib/backuppc. If growing full is a concern, then you'll want to migrate everything
to a larger disk, or use a filesystem like LVM that will allow you to grow it into multiple
disks (but can be risky) or move to a RAID setup that can provide both the ability to
grow and redundancy.

In short, don't use symlinks anywhere under /var/lib/backuppc unless they point to
something on the same filesystem.