why is my external HD bay less effective at reading a failing hard drive?
 

I know my backup hard drive is starting to fail, because the external USB bay sometimes can't access it; and when it does, I sometimes (not always) hear the characteristic ticking . Until I can get around to ordering a replacement, I knew a trick that often helps: put the failing hard drive in my laptop's internal bay and run the main hard drive from the external bay. The internal bay accessed the drive more easily. I just wondered why an external bay has more trouble. I vaguely supposed it has to do with the USB connection having some inherent limitations. Can anyone explain it better?

Does the USB dock have a power supply of it's own? If not I'd suspect that the power draw is higher on the drive so it's failing more when only supplied with USB power at around 5A.

No...it's a 2.5" bay that only draws power from the USB port. I'll test the hypothesis by plugging it into my 3.5" external bay that has its own power supply.

Oh, darn, I meant 5W but yes.