SATA disks suddenly stop working

I've been having lots of problems with SATA drives attached to higher
port numbers, namely ata5 and ata6.

I was installing Linux under qemu today and it had been running for
several hours and had installed multi-gigabytes of data when qemu
just stopped.

I noticed that all I/O to the disk had ceased.

Doing "atacontrol reinit" on the port (ata5) resulted in a message
that the device was not configured, which was patently false since
qemu had just been merrily writing to it.

This with a kernel made from sources updated today at about 2 PM (GMT+1).

I've also seen problems with a disk attached to ata6.  It just sort
of disappears after a while.

Disks attached to ata2, ata3 and ata4 don't exhibit any problems.

Is anyone else seeing this weird behavior?

---
Gary Jennejohn