Missing SATA HD

Strange goings-ons.

I have two hard drives, both XP PRO, both bootable. One is IDE
Primary Master, the other is SATA. I can select either one manually
at boot (F8) by selecting the drive I want and it will boot up fine.

If I boot to the SATA drive, the IDE drive shows as D drive and I can
access all its contents. However, if I boot to the IDE drive, the
SATA drive is nowhere to be seen, with D drive left unused (in MY
COMPUTER and WINDOWS EXPLORER).

Any ideas anyone as to what has gone wrong?

Thanks

Duke