Xorg detaches one of my hard drives

[Nathan Butcher]

I started up xorg on my Gigabyte GA-G33-DS3R motherboard, which contains
6 drives on the ICH9 SATA interface:-(ad6,ad8,ad10,ad12,ad14,ad16)

I Didn't expect to see this kernel message mixed amongst the xorg
startup messages though:-

subdisk16: detached
ad16: detached

...and sure enough, it was no longer in /dev

Any clue why xorg feels it has the right to kick one of my drives off
the system? Note that the same thing happens whether ICH9 is in RAID or
(now working) AHCI mode.

IRQ issue?