I have an external USB powered hard drive that's been sat unused for a good year or more and has worked fine and dandy in the past.

It's never been knocked or exposed to temperature extremes or any of the usual drive-killing stuff.

Now that I want to use it again, it's incredibly slow and takes ages to do anything (show up in Explorer and/or move files).  In fact when I plug it in, Explorer hangs for about 30 seconds.  Do these things just fade away and die? 

It tends to be the interface controllers on them are the things to go, they are bloody rubbish to be honest. If it's still within warranty I'd get the data off and RMA it before it gets too bad to use. I wouldn't buy another external HDD these days, I'd rather have internal drives.

Hmm.  Plugged it into a USB port on the back (direct to mobo) and it seems to be ok now... might just have been an underpowered hub thang.

Cheers.

If it's not a DC powered unit I think you sussed it You mentioned powered in the OP but I didn't realise you meant powered from the USB.