If they are hot-pluggable drives (in a setup like that they should be) there will be one disk with a red LED showing on the front panel instead of a green LED.
NB - Often, you can pull a bad raid disk out, clean the contacts put it back in and it will start working again, but it will likely die again a few days later. It can get you out of trouble for a few days if you have to wait for a replacement.
If they are all showing green lights then the problem may be intermittent and the drive keeps going back into "rebuild". Then you need to use the raid diagnostic on the machine via remote login to identify the drive ID. Get the serial number for the bad drive from the diagnostic and match it with the serial number on the front panel of the hot-swap bay.
If the diagnostic software can't tell you which drive is failing then either you have been given wrong information or the diagnostic is wrong. Cross-test with another diagnostic package.
Best I can do from here. Good luck.
>me thinks I have done this before a few times <
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