I had a similar issue with a USB pen drive.
I deleted the /media/.hal-mtab.lock file but it had no effect.
However, this is what I did to resolve it:
1. Launched 'gconf-editor' and browsed to 'apps / nautilus / preferences'
- unchecked 'media_automount'
- unchecked 'media_automount_open'
2. Launched GParted to find the id of the location of the device (/dev/sdf1)
nb this took up to one hour to complete
sudo mount -t ext3 -o rw,users /dev/sdf1 /media/usb-backup
After that the device was available as normal. I unmounted with:
and removed the device
sudo umount /media/usb-backup
Then re-enabled the Nautilus preferences unset in step 1.
Plugged in the device again and it was found as normal.