Charles,

>>Thereafter nothing happens, fsck_ffs seems to be blocked on physio,
>>probably in the driver.
> 
> There are conditions under which an ATA disk will simply stop responding to 
> any commands.  The driver should notice this and start timing them out, 
> though.  How long did you wait?

Not hours, but 10 mn at least. Nothing more. No kernel messages, 
fsck_ffs still blocked in physio.

The first time, I had the absurd idea of connecting the faulty drive on 
the same IDE bus as mine: that was a grave error: the IDE bus was hanged 
busy, so there was no way I could access my own disk. When I connected 
each on a different cable, though wd1 was hanged up, the other carried 
on working normally.

V.