Disappearing External Hard Drive

Hi All,
I'm just about at wits end here. I've got an external HDD (Cavalry 2 X 500
GB = 1TB) that I usually plug in via e-SATA. It is currently setup in a
SAFE33 configuration. It was working great on my old XP machine, and when I
changed that machine to Vista Business (same everything, just plugged in a
different boot disk and unplugged the old one) it continued to work great.

Then I did the following two things:
1) Tinkered with some permissions to allow only administrators access to
some backups on the eHDD.
2) Went back to my old boot disk and tried to find the drive in XP.

When I did this the drive was not visible at all in XP. Now it is not
visible in Vista either (not under Computer, Disk Management, or even BIOS).
It is also not visible on my laptop (it had been previously). I have tried
both e-SATA and USB connectors in multiple ports.

When I turn the drive on while either OS is running the machine freezes
about 1 out of 5 times. Other times the eHDD turns on and I hear the disks
spin up to speed, but the drive access LEDs never come on.

Don't know if this is related or not but I am also unable to load XP now, it
stops on a driver called SPTD.sys.

Please tell me there's a possible solution other than the drive being cooked.

Thanks!