Sequel to our 40S troubles

Some weeks ago, as some of you may recall, I was having difficulty with
our 40S refusing to IPL, and giving a mysterious SRC that didn't show up
in any manual. We eventually got it back up, but never did figure out with
any certainty what the problem was.

Now, we have a concrete problem: it's lost a disk drive. Worse: the
missing drive is half the load-source mirrored pair. Fortunately, we
mirror everything.

All of the drives are 6606s. We started out with 8 of them; we're now down
to 5. We've got plenty of spare 6607 drives for our production box, but we
don't have one bloody damned spare 6606 that hasn't gone bad.

1. Will a 40S take a 6607? Can it be physically shoehorned into a drive
bay? Will the system recognize it?

2. Does anybody know where I might find some known-good 6606 drives?

--
JHHL