my external drive does not appear in my computer

My external hard drive - (separate casing and drive, so unsure of model number, etc.) - plugs in and turns on. However the computer does not register it and see it in Explorer. It does pop up the 'safely remove hardware' icon. So it registers it, but it doesn't!

The system is XP - and the computer is a few years old, but doesn't have other problems other than not enough ram to run fast.

Replies would be really appreciated - advice on possible fix-its or reasons why it won't work.
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Thank you!