after installing ubuntu-server, HDD's are not detected in AHCI mode

[scar]

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i have two identical servers with identical hardware.  the servers are
sun fire x2250's with two 1 TB hard disks.  on server #1, i installed
ubuntu-server a couple weekends ago.  in the BIOS i configured the disks
to use AHCI mode.  everything works great.  i created two RAID-1 md
devices.  on md0 i formatted ext4 and put /boot.  on md1 i put LVM with
logical volumes for / and swap.

then, last weekend, i did the same thing (AFAIK) for server #2.  only
this time, after rebooting, the hard disks are no longer detected; the
BIOS just hangs at "auto-detecting SATA PORT 0".  if i change, in the
BIOS, the disks back to IDE mode instead of AHCI, they get detected.
isn't that strange?

i tried to clear and re-setup the BIOS, didn't help.

when i put the disks from server #2 into server #1, server #1 doesn't
detect them either.

i had another pair of disks from identical server #3 that i decided to
try out.  again, initially, they are detected fine in AHCI mode; i go
and setup ubuntu-server on them similarly to the way i did on server #1;
upon rebooting, the BIOS hangs detecting the disks.

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