HDD light always on

My problem is that sometimes my HDD light would come solid on wo/ data access happening. This is a WD Caviar drive. Related symptoms:
- intermittently windows (XP or W7) no longer able to see drive.
- reboot would sometimes fix problem, not always.
I replaced the motherboard and SATA cables, without success. I even replaced the HDD and still had the problem.

Finally I think I found the real cause. The power supply, or the power cable. I swapped the cable to the HDD with the cable going to DVD (also SATA) and the problem seems to be gone. Not sure if it was a bad cable or supply noise. Either way, it is time to replace the supply.
This note is posted to hopefully give other people something else to try as well as to see if anyone else has experienced the same issue.