If this external drive is powered directly from USB port a common problem with external hard drives is that the USB power is weak or the cable to the external HDD should be replaced with a heavier watt/amp load cable so it can handle the current and power the HDD properly.
With all the mini USB cables that come with various devices MP3 players, Camera's, PlayStation 3 Controller, etc its possible to use a cable that came with a different device than the external HDD originally came with and end up with problems. I have 2 such cables that cause my external 2.5" HDD to click that came with something else but were in reach when wanting to connect the external HDD.
The better cables have 2 USB connectors at the end with mini USB connection at the other end so adequate power can make its way to the small external HDD so its adequately powered. So here is the cable that I use with my external 2.5" HDD
http://www.newegg.com/Product/Product.aspx?Item=N82E16812200791&cm_re=mini_usb_y-cable-_-12-200-791-_-Product*There are a few different USB connections out there so if replacing the cable and buying a new one be sure you selected the correct USB connection, Micro, or Mini, etc.
If this is a unit that came with a separate power supply and while powered its making clicking noises and acting sluggish this is not a good sign and I would make a copy of all the data on that external HDD to another drive asap to avoid total data loss!
Even if its a smaller 2.5" external HDD that is powered off of USB power, I'd also back up your data to another drive asap in case that drive is failing as well.
For my most important data i keep it backed up on 2 external hard drives that I create a mirror image of as well as save on DVD's etc if it makes sense to do so. * I have been bitten by data loss in the fast and would hate to lose some specific data that could never be replaced etc.
