[geeks] weird power supply behaviour

[Mouse]

I've just had some weird behaviour I don't understand.

The scenario: power supply, peecee motherboard (fairly old - it's a
Celeron 400), and two disk drives, A, and B.  The PS is way over spec
for the MB and disk; the (dead) PS that came out of the machine is
rated 250W and this one is rated 400W.

With the MB and disk A connected, disk B not involved, everything works
fine.

Swap out disk A for disk B and the PS apparently goes into overload
shutdown - it powers up long enough for fans to spin, but then drops
power almost immediately.  Standby power remains up; I have to kill
mains power and wait for standby power to drop to reset it.  This
occurs even if I unplug the data cable between disk and MB.

Okay, so that disk is just failed in sokme way that looks too much like
a short to the power supply, I figure.

But no.  I put disk A on and get it running enough to display stuff on
the screen, then plug power into disk B live - and disk B spins up
fine.

So I reassemble the disk-B machine, only I don't plug powe rinto disk
B.  I let the MB spin up enough to start displaying, then plug power
into disk B.  Power supply shuts down.

The PS is willnig to power the MB and both disks, but not MB and disk B
alone.  Whether data cables are connected or not appears to make no
difference, so it's not something being incorrectly grounded through
the data cable.

Any theories?  I have no theory to explain this.

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