>This DDRS tends to spin down every now and then "out of the blue".
>Sometimes I see comments in the syslog, sometimes the machine simply hangs.
>I cannot tell whether the drive first spins down and up again or whether
>some rude action from the driver forces the drive into a cold reset.

well... you are not crazy, i have seen this too with some similar ibm drivers
(some older 1G and 2G models, i forget the model numbers but they were
definitely ibm)  i really dont suspect the problem is with the ncrscsi
driver (since i am using a q800/q840/q950, and macos running on a 7100)

i am almost positive this problem is heat related:

. my q950 at work never spins down (my office is 66-68F)
. on the q800 and 7100 at home i have heard the drive spin down/up
  (seen it in the netbsd logs sometimes, and macos says 'this drive
  has problems')
. i added a fan to the rack at home (since the q800 doesnt have
  a case because is hotwired to a pc power supply) to circulate
  the air and havent seen a problem since (everything is in the
  rack: printer, hub, 7100, q800, modem so i imagine it gets
  warm in there)
. i didnt have this problem at home until it started to get warm
  recently (+100F outside in some cases and i set my thermostat at
  home to 80F)

i wonder if the ibm drives have some sort of temp sensor onboard?