3.5" slim floppy goes wild when turning off the computer

[eto]

I attached an external 3.5" slim floppy to my CPC 6128. It's connected through a 26flat ribbon/34 floppy adapter and powered by an external USB adapter. 

Everything is fine, works as expected, it writes and reads data. But when I turn off the CPC while the drive is still connected, the head starts moving back and forth. When I turn on the computer again, it stops, but the disk is no longer readable. After formatting it, I can use it again. There is also no issue if it's powered before I turn on the CPC. Just when turning off the CPC, the head starts moving and crushing the data.

Any idea what is causing this?

[pelrun]

For one, never leave a disc in the drive when turning on/off the CPC - a voltage spike on the read/write head will happily mess with whatever bits happen to be right under it at the time. And if the drive is externally powered, you should be turning it off before the CPC.

As for why it's moving the head about - once it's off, the CPC is no longer driving the signals to the disk drive and so they're floating. That means the wires in the cable to the drive are now effectively antennas, and they pick up all sorts of noise, which the drive thinks are legitimate control signals. Cue lots of thrashing about.

[GUNHED]

The CPC is off, but the FDC still has enough voltage to send ghost commands to the floppy. Maybe due a power leak from the floppy into the CPC. I don't know the cable.

However, I never has such a phenomenon.

What I can reproduce: Switch the power of an HxC B-Drive off and the CPC will destruct any 3" inside drive A. Sadly.