Hi Robert,
it is an old, but a rock solid piece of equipment.
Due to the fact that 2TB HDs are no longer produced, it is dying slowly.
So, in case you cannot replace the fan or repair the contacts in the enclosure, it is gone.
I would try to clean the contacts of one fan module, or try to switch all of them several times.
If this does not work, I would try to buy a spare fan module via ebay.
If this does not work, I would retire the Isis.
cheers
Rainer
factstory
Rainer Wirth
phone_0049-177-2156086
Mac pro 8core
Adobe,FCP,Avid
several raid systems