ZFS mirror boot: cannot boot from 2nd disk

[Scott Johnson]

I installed 8.1-RELEASE onto two identical drives in a mirror, following the 
"Installing FreeBSD Root on ZFS (Mirror) using GPT" guide here: 
http://wiki.freebsd.org/RootOnZFS/GPTZFSBoot/Mirror

I can boot from either disk by changing the boot priority in the BIOS, so long 
as both disks are connected. I swapped the sata ports, and I can still boot from 
either disk.

I can disconnect drive #2 and boot from drive #1 just fine, whether drive #1 is 
plugged into sata0 or sata1.

However when I disconnect drive #1 and try to boot from drive #2, on either 
sata0 or sata1, I get errors:

error 1 lba 32
error 1 lba 1
error 1 lba 32
error 1 lba 1
error 1 lba 32
error 1 lba 1
error 1 lba 32
error 1 lba 1
error 1 lba 32
error 1 lba 1
error 1 lba 32
error 1 lba 1
No ZFS pools located, can't boot

It looks like a hardware problem, but then why does disk #2 work when disk #1 is 
connected?

(Note, if both are plugged in, everything works fine; zpool status is clean, 
zpool scrub is clean, SMART status is good after long self-test.)