Fluctuations in activity and not actually turning on/off? The hard drive is usually quite, so you typically won't hear it unless it's under a high level of activity, heavily fragmented, or damaged. A few things you could check the next time it happens include:
1.) Open My Computer and see if the drives are still listed/accessible.
2.) Use a hard drive monitor such as Steller Smart to keep an eye on it.
3.) Leave your case open, touch the metal frame a couple times, then feel the hard drive(s) to feel if they're still spinning.
4.) Check the S.M.A.R.T. results with a tool such as DiskCheckup.
5.) Run chkdsk, the Windows tool/command, on all of your drives.
If a hard drive is actually turning itself off, it could be detrimental. Obviously data loss is a possibility should it turn off in the middle of writing, and if it's a power problem it could do physical damage. Thus, it's definitely worth looking into, just in case.
John
I just went to a dual boot system this past week. XP runs fine but when I go into Vista (Home Premium) from time to time I can hear what sounds like one of my HDs shutting off and turning back on randomly. There is no interruption with my graphics or whatever program I'm running so apparently it's not the graphics card or the SATA drive I'm running Windows VISTA from. I've got XP on a separate IDE drive. Two IDE HDs in all for XP (primary is FAT32, secondary is NTFS). Third HD is the SATA for VISTA.
Specs: eVGA 680sli, Intel Core2 Q6700
BFG Geforce 8800 GTX OC, Corsair 2gb mem
PSU - Antec Trio 650w
Thoughts? If my system isn't crashing, should I care? If an IDE HD is shutting down/turning on like that would it harm it if it continues?

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