proper care of external drive - should I keep it powered up?

I have a WD drive in an ADS enclosure. Is it harmful to the hard drive to
shut down and power it back up often (couple times a day) or should I leave
it up and running as long as the PC is up?

The reason I ask is that I tried putting in an older drive into the external
drive case (after it had been sitting unpowered for about a month ) and it
did not want to be detected by XP. It kept making clacking noises. Note that
it did have some problems awhile ago (it had crashed once in windows and had
bad sectors on it). I was wondering if it sitting without power and not
being used for a month caused it to lock up now. Is this something likely to
happen to any hard drive or is it just because this drive had problems to
begin with?

--
Phil Tomaskovic
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