MFM drive gone nuts

Hello!

My C900 arrived today. After replacing the broken PSU, the computer
appears to be working except the hard drive. The drive developed a
schizophrenic personality and is refusing to talk to the controller.
Instead, it has its own agenda as evidenced in this video I made:

https://www.youtube.com/watch?v=gRXC9SXme-4

Please note that this is how the drive acts _without_ the controller.
Only power plug is attached. Yet, the drive seems to perform various
tasks by itself - the disk spins, the heads apparently goes back to 0
position. And then bam - watch at 0:29 as the head starts moving
randomly and rapidly. I lfet it running for several minutes and it
didn't stop. I wasn't aware that MFM disks could do that on their own.

Remember again that this is without the controller attached. Attaching
the controller, the controller to the computer etc. has no effect. The
computer is unable to communicate with the drive as it performs its head
dance - all operations result in timeout.

Do you possibly have any idea what is going on with the drive? I thought
that maybe it doesn't like the voltage from the CBM710 PSU, but I doubt
it - the whole computer runs on it flawlessly (including the floppy
drive). Or maybe something was broken when I attached the CBM610 PSU and
learned that it is too weak to spin the drive? But in any case, this
would be very strange symptoms of a power failure.

Or is it some kind of self test? If so, why is it running and how to
disable it? There are no visible jumpers on the drive that could
possibly be used. I would appreciate any insight...

Regards,
Michau.

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