Is there *ever* any reason to downgrade the mode for *any* reason
other than a UDMA `transfer error'?  Under what condition would, say,
an `uncorrectable data error' or `id not found' be resolved by
downgrading?

As far as I can tell, all this does is cause a surface analysis test
to take approximately 6 times as long on my machine, because it steps
all the way down from UDMA mode 4 to PIO mode 4 as it encounters bad
spots.

This has also bitten me in a different form with CD-R drives -- I'll
accidentally try to read a blank disc or something, and it will
immediately downgrade modes.  I have to reboot to get it back.

This is annoying, and I think it needs to be fixed.