I have one of those "external" external hard drives that isn't an exclosure per se but is more just an open dock to vertically slide an exposed SATA hard drive into. I have it plugged into my Mac Mini via USB. If I turn it on, it connects fine. Everything works. An hour later or even a few hours later, it's still fine. However at some point past a few hours, such as overnight, the hard drive de-mounts from OS X 10.6.8 that's on the Mini. I've turned off the Power Management mode that allows putting a hard drive to sleep and that didn't help.
What's weird is the power is still on to the hard drive "enclosure" but it's not recognized by the computer anymore. If I turn the hard drive off and back on again and also unplug the USB and back in again, then it's working fine as intended. Any idea what might be causing this?

Any help will be apprecited.

I didn't find the right solution from the Internet.

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Thank you.