A have a 500GB Transcend StoreJet which turns off after a while when it's not in use. The hard drive runs perfectly though – no issues.
It can't be related to my USB ports since my other external HDDs run fine all the time, except this one when it's not in use. If I unplug the USB and plug it back in, it works fine until it's not in use.
Best Answer
This feature is called by marketing "green" or "smart" or "Independent Power Management", and it puts the HDD to sleep when not in use. It can be removed using firmware tools from the manufacturer.