`cannot create hard link` during snapshot

[antocuni]

I am not sure whether this is a bug in the code or in the documentation (or both 😅).

This is more or less what I did:

$ mount /dev/sdb1 /data
$ mount /dev/sdc1 /mnt/external
$ rsync-system-backup /data /mnt/external

The "backup" step worked fine. Then the script tried to create a snapshot under /mnt/2017-12-05 15:24:47/, and I get tons of errors like this, since /mnt/external and /mnt/2017*` are on two different disks:

cp: cannot create hard link '/mnt/2017-12-05 15:24:47/...' to '/mnt/external/...': Invalid cross-device link

In general, I'd say that creating the snapshots in the parent directory of the destination is a bit confusing: or, at least, it's not what I'd expect. Maybe it would be better to create them under a special /mnt/external/snaphosts folder, or similar?