"I have tested the same template on an X61, X60, T60, and T400, with no problems in AHCI mode, so I don't think its a driver issue".
Nothing that you say proves that it's not a driver issue. Try a manual diskpart (e.g. list disk command) - that will show beyond doubt whether or not WinPE can see the hard drive.
Diskpart List Disk clearly sees the drive as active and healthy. I can also browse the drive. Doens't make a different for this if SATA is set for compatibility or AHCI mode.
Just fails Step 1 in the templates as noted above.