Authentication problems with Plextor PX-716 drives

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I have two Plextor PX-716 drives (one ATA, one USB), both of which have
authentication problems. The ATA unit is brand new, and the USB unit is
at least 6 months old. From what I can tell by looking at the source,
authentication fails in GetBusKey() because it can't determine which
variant the drive authenticated with. I looked at what the drive
returned for key1 and found that all 5 bytes were set to zero.

It seems that this only happens when the drive has spun down after
idling for a bit. Before then authentication works fine. I've set the
region with regionset, but it didn't make any difference.

I'm running linux 2.6.12.5, but I've had this problem with other 2.6
kernel versions as well. I don't have this problem with the drive in my
laptop on the same kernel version. I looked in the kernel source to see
if it was somehow mangling the challenge or key, but everything appears
to be passed straight through to the drive.

Any ideas? I've heard that some drives made by AOpen have a similar
problem, and that some closed-source windows decrypter programs have
implemented workarounds for this.

I haven't subscribed to the list, so please CC me when you reply.

Thanks.