USB Hard drive only recognized sometimes...

[b.libbey]

I have a Seagate FreeAgent Pro 320GB and a Sun M20 or something like that -- an x64 worskstation running 32-bit Gentoo (2.6.22-gentoo-r.

I have a nightly backup script set to rdiff a bunch of files onto the external USB hard drive. This script mounts, backs up, then unmounts the drive. It works fine once or twice, then the drive is apparently removed from the USB bus or something.

It works fine again after disconnecting and reconnecting the hard drive.

/root/rdiff-backup # lsusb
Bus 001 Device 001: ID 0000:0000
Bus 002 Device 008: ID 0430:00a2 Sun Microsystems, Inc.
Bus 002 Device 002: ID 0430:100e Sun Microsystems, Inc.
Bus 002 Device 001: ID 0000:0000

That's just the USB keyboard connected to the machine.

My question is this: What kind of software event would cause a USB hard drive to be unrecognized by the OS until it's reconnected?

Thanks