I've discovered the hard way that a cheap 2 port PCI express card with a JMicron JMB363 chip on it hard locks FreeBSD 7.0 amd64 - usually within a minute of starting my disk testing (thrashing) program. This is a true hard lock: everything frozen, no keypresses work, even the reset button does nothing for a few moments... and then the computer powers off! (I've never seen this behaviour before, I presume it must be some failsafe circuit that realises things are FUBAR and the only option is to shut down.)
I suspect it's probably an AHCI quirk. The card's BIOS doesn't have any option to enable or disable AHCI in hardware, so instead I tried using FreeBSD's atacontrol to force the drives on that controller into SATA150 (and hopefully non AHCI) mode. It refuses to make any change, stubbornly reporting that the mode is (still) set to SATA300.
As a last resort, I examined and hacked the kernel source:
/usr/src/sys/dev/ata/ata-chipset.c:
- { ATA_JMB363, 0, 2, 1, ATA_SA300, "JMB363" },
+ { ATA_JMB363, 0, 2, 0, ATA_SA150, "JMB363" },
On boot it's still reported as an AHCI compatible controller, and the attached drives are reported as SATA300, but it's no longer crashing the comp: the disk testing program successfully ran for over an hour, which is about a hundred times longer than it's previously lasted.
One other option that occurred to me was to jumper the drives to force SATA150 mode (first thing I do with a new Seagate is pull out the factory jumper), but unfortunately I had no idea where or if I'd stored the minature jumpers.
The software fix seems to be working. Fingers crossed.
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