The past couple of days I've been getting this error printed to all consoles that are logged in:

Code:
kernel: ------------[ cut here ]------------
kernel: kernel BUG at fs/inode.c:174!
kernel: invalid opcode: 0000 [#1]
kernel: SMP 
kernel: CPU:    0
kernel: EIP is at destroy_inode+0xc/0x45
kernel: eax: 00000001   ebx: e0f6b640   ecx: 00000003   edx: e0f6b734
kernel: esi: e0f6b640   edi: 0000552c   ebp: e07b7f00   esp: e07b7edc
kernel: ds: 007b   es: 007b   ss: 0068
kernel: Process umount (pid: 29091, ti=e07b6000 task=dff98aa0 task.ti=e07b6000)
kernel: Stack: e0f6b648 c016f228 c31d4074 d1dabdf8 0000c2c8 c31d4074 c016f4c2 00000000 
kernel:        cec7fa98 e0f6b4b0 cec7fa90 c31d4000 d2f1ea5c f8dd0600 00000000 c015f889 
kernel:        df875880 c31d4000 00000000 c015f953 c31d4000 f8dd05e0 c015f9fb f6896c40 
kernel: Call Trace:
kernel: Code: 16 c0 ba 03 00 00 00 89 f0 e8 c2 0d 11 00 58 eb 08 b0 01 86 05 dc d7 2c c0 89 d8 5b 5e 5f c3 53 89 c3 e8 34 c5 fe ff 85 c0 74 08 <0f> 0b ae 00 35 c9 29 c0 8b 15 a0 66 37 c0 89 d8 ff 92 8c 00 00 
kernel: EIP: [<c016ecc4>] destroy_inode+0xc/0x45 SS:ESP 0068:e07b7edc
This error leaves the server somewhat crippled. Postfix stops delivering mail and a 'shutdown -r now' has no effect.

I do nightly backups to two iOmega REV drives (USB), are mounted using the udf filesystem and I think they might be the problem. I'm hoping it's not a problem with the FS on my hard drives.

I'm running Linux version 2.6.18-5-686 (Debian 2.6.18.dfsg.1-17)

How should I proceed on debugging and fixing this?