I've noticed this before, but today I've sat down and tested. When two
drives are in heavy use on a CATS it completely locks. For instance if
either of the two following commands are run separately, everything works
fine. If both are run at the same time the machine will completely lock.

dd if=/dev/rwd1c of=/dev/null bs=10240k
dd if=/dev/rwd2c of=/dev/null bs=10240k

This happens with different motherboards, and with kernels 1.3F, 1.4P and
1.4ZD. The drives can be on the same or separate IDE buses.

The above examples are somewhat contrived, but even doing a tar from one
disc to another is sufficient to eventually hang the machine. 
Alternatively, a real world situation where user areas on one disc
and the OS is on the other proves to be a very unreliable system. 

--
Dr. Stephen Borrill