I do not know if this will help, but I had a similar problem using Ghost 2003.
Both my OS drive (XP Pro, 80 Gig) and my Slave 200Gig are IDE connected.
The OS drive is NTFS and the Slave FAT32.
In addition to storing Ghost backups onto the slave, I also store all my rubbish there.
As a practice, I always save two or 3 Ghost backups as I back up every month. Should my most recent backup fail, I could then resort to the next (oldest) backup.
As time progressed my Slave got more full. Then a few months ago (that is when the problem presented) I made my regular monthly Ghost backup, only to find that it would either fail 3/4 of the way through the actual backup or when I tried to verify the backup (can't remember which one exactly) .
Then by accident after deleting one of my older Ghost backup from the slave thereby creating more room on the Slave, the backup went through with out any problem.
This failure I replicated in the following backup which I tested out a few hours later. I repeated the test again the following day and sure enough the same problem each time.
What I noticed was that when I made more room on the Slave by deleting a Ghost backup, (approx 23 Gigs per backup) all went well.
This struck me as strange as for all intense and purposes I still had plenty of free space on both the drives.
The only conclusion that I can draw here (and it is only a guess) is that Ghost must need a certain amount of free space on the target drive for temp files during the backup process.
Just as a further point, when I use Ghost 2003, my box (rather old) does not recognize USB connected external drives nor does it recognize partitions. That is why I now use Ghost11.5.
When I do use Ghost 2003 I boot from a floppy to DOS.
Thinking hurts my head that's why I live in West Australia