USB External Backup Disk and Vista Pro

Hi

We use a USB external hard disk to backup to from 2 Vista Pro PCs on a wired
LAN. All works OK until such time as the drive is disconnected from the
main PC. The drive is taken off site at the end of each working day and then
plugged back in the next day. With it connected I have set all the backups
no problem and have also "Shared" the disk so that the 2nd PC has access to
it. I think what is happening is that the share is lost when I disconnect.
Is there a way to overcome this issue or is there another explanation? I
have to say I am more than a little puzzled by this as I used the same setup
with our old XP machines.

Kind regards
Grandad