Disappearing drive on Xserve

Intel-based Xserve running OS X Server 10.5 with current updates installed. The system has two drives in the drive bays – a 1TB drive containing the system and some data and a 500GB drive just containing data. It’s installed in an office environment with a network of mostly Macs and a few PCs. There are shared folders on both drives with the data on the 1GB drive being accessed much more frequently than that on the 500GB drive.

From time to time, the 500GB drive will disappear from the Desktop on the Xserve and the shared files on it will not be accessible from the network. When this happens it no longer appears in Disk Utility. Using the ‘lsof’ command doesn’t show any open files on the drive. The drive does appear to be mounted when checking for its presence via command mode (for example it is there as a device in ‘dev’) but a regular umount reports it is locked. It can be forced to umount but this doesn’t do any good. The only solution I’ve found is to Restart the server when it appears again on the Desktop and can be accessed from the network.

I don’t think that it is something to do with the drive itself as this has been replaced. The only software in use other than OS X Server is Retrospect Workgroup version 6 with latest updates that is used to make daily backups. The backups are on to a set of external FireWire drives (one for each day) that are mostly left attached and powered-up although weekly and monthly backups are kept off-site. The FireWire drives aren’t chained together but connect through a FireWire hub.

I’ve a suspicion that Retrospect is causing the problem but I haven’t any evidence to support that theory.

Any suggestions as to what might be happening and how to prevent it in future will be welcomed.