Mounting a powered-down HDD renders system unusable

[Mike Harding]

On a 5.4-pre system as of the last few days, If I mount a spun down hard
disk (one I am using only for backup):

# mount /backup/usr

I get

ad1: TIMEOUT _ READ DMA retrying (2 retries left) LBA=7340273
ad0: WARNING - removed from configuration
ad1: WARNING - removed from configuration
at0-slave: FAILURE - READ DMA Timeout
mount:/dev/ad1s1g : Input/Output error
ata0-slave: timeout state=0 unexpected
initiate_write_filepage: already started
initiate_write_filepage: already started
(repeat until reset is pushed)

Nothing works because all of the disks are unmounted.  Can't even shut
down.

This worked great in 4.10, and also 5.3 (where the initial mount
generated an error, but I could do the mount again after the disk spun
up).

Anybody else get the same thing?  I'll open a bug report if so...
-- 
Mike Harding <mvh at ix.netcom.com>