ad2: 152627MB <WDC WD1600BB-00GUA0 08.02D08> at ata1-master UDMA100 GEOM: ad2: geometry does not match label (81h,63s != 16h,63s).
This appears to be a harmless message and can supposedly be fixed with:
bsdlabel -e -A /dev/ad2
And it did. After manually fixing the heads, I could mount it. Ran an fsck:
to make sure nothing was screwed, and it wasn't.