Joakim Aronius wrote:
quoted text
> I have an old home server which ran out of disk space 
 > I added a big disk over USB which I use for
 > backup (mounted on /backup).

Well don't do that!

Mount under /usr/backup, or /var/backup, or /tmp/backup or whatever!

quoted text
> Is this due to my ancient HW, crappy USB controller card, or is USB mounted disks just less stable? 

Even if the externally powered USB disk never fails, still there is the 
chance that the two independently power supplied boxes get out of sync, 
for instance in the event of a power cycle...
You power cycle both of them: do they come back again in time to pick 
each other?
(I have a machine with a USB disk where this NEVER happens, when the 
bootstrapping OS tries to mount the USB disk, the disk is always 
spinning up or whatever but it's not ready yet so the mount fails)

-- 
Mauro Rezzonico <mauro@ch23.org>, Como, Italia
"Maybe this world is another planet's hell" - H.Huxley