Пропадает диск из вывода lshw

dmesg | tail сразу после пропадания что показывает?

ничего:

scsi 8:0:0:0: Direct-Access ATA SAMSUNG HD103UJ 1AA0 PQ: 0 ANSI: 5
sd 8:0:0:0: [sdb] 1953523055 512-byte logical blocks: (1.00 TB/931 GiB)
sd 8:0:0:0: Attached scsi generic sg2 type 0
sd 8:0:0:0: [sdb] Write Protect is off
sd 8:0:0:0: [sdb] Mode Sense: 00 3a 00 00
sd 8:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
sd 8:0:0:0: [sdb] Attached SCSI disk
EXT4-fs (sdb): mounted filesystem with ordered data mode. Opts: (null)

А если dmesg | tail -100 ?
Что говорит считающая md5sum программа? Как подключён жёсткий диск?

FUA/DPO is scsi parameters.

from man sg_read

dpo=0 | 1
when set the disable page out (DPO) bit in SCSI READ commands is
set. Otherwise the DPO bit is cleared (default).

fua=0 | 1
when set the force unit access (FUA) bit in SCSI READ commands
is set. Otherwise the FUA bit is cleared (default).

from http://en.wikipedia.org/wiki/SCSI_Read_Commands

Disable Page Out (DPO) allows the initiator to warn the target that the data being read is unlikely to be requested again soon and so is not worth keeping in the target's data cache.
Force Unit Access (FUA) tells the target to fetch the data from the media surface and to not use a cached copy.

turning on AHCI in the BIOS and compiling in AHCI support does it instantly, because the AHCI drivers try to detect before the piix drivers.