I have a series of USB harddrives connected to a Centos5 box. Had a power cut today and now one of the drives won't remount (all others do). They are all the same make and model.

When I try to mount it says 'mount: special device /dev/sda1 does not exist'

I have checked dmesg and get this:

SCSI device sda: 1953525168 512-byte hdwr sectors (1000205 MB)
sda: Write Protect is off
sda: Mode Sense: 34 00 00 00
sda: assuming drive cache: write through
sda: unknown partition table
sd 5:0:0:0: Attached scsi disk sda
sd 5:0:0:0: Attached scsi generic sg0 type 0
usb-storage: device scan complete
EXT3-fs error (device sda): ext3_check_descriptors: Block bitmap for group 880 not in group (block 0)!
EXT3-fs: group descriptors corrupted!


Help, it's an ext3 filesystem. Really need it.

The drive is having a problem, and can't mount. Either it's gone bad, or you need to fsck it. Did you get any messages about needing to run an fsck manually on the drive?

Try:
Replace the /dev/sda1 with whatever the mount statement is in your fstab file. And you can try 163840, if 8192 doesn't work, for a replacement superblock. The -b option should only be used if the primary superblock is corrupted. If you can't fsck it, the drive is dead. Get a new one, and reload from backup.
Then you should make sure you have backups, especially with USB drives.