This is a real conundrum.
I have a late 2009 iMac,
and use 2 external FW drives and a portable USB.
I work in Avid and After Effects.
Whenever I use all three drives simultaneously,
my Mac starts acting funky and eventually stops
recognizing the FW drives. I run Disc Utility
and am told main drive is corrupt
and needs repairing.
That works but this is becoming areal pain.
I have had the iMac in for authorized service
and they can find nothing physically wrong
with the main drive! They have run every overnight diagnostic
in the book.
Is there some issue with mixing USB and FW?
I am certainly unaware if so, but this seems
to be the culprit. The USB gets transported back and forth
to/from work and is formatted to work with both Mac
and PC for those reasons.