netdokter
Promise supplies two drivers, irrespective of how you set the BIOS, if you load the ATA driver Win2k will see two disks, if you load the Array driver it will see one disk. I susspect you have both drivers in your system and a system glytch has made it reload the ATA driver. If you have a recent system registry hive backup you could reload that, or you could reload the Array driver.
If you want to recreate a mirror set from one good drive you can do that safely. Connect ONE drive ONLY to the array PATA connector. Set the BIOS to RAID, power the system up and go into the setup utility - ctrl F.
Write down the drive identifier and the channel number of the drive. Power down the system and connect the second drive. Go back into the Array utility.
You will now see two drives, but now you will know which is which. select recreate array, you will then be given the option to mirror one disk from the other. Select the disk to mirror from, you will now understand why you did step one :-)
Say yes and go to the pub. When you come back you will have a mirror pair. I have done this several times so I know it works. The time taken is roughly 100GB per hour.
Good luck.