Sudden slowdown on file transfer from External HD to Internal

So I recently had to do a factory reset on my desktop computer, and have been busily reinstalling and configuring programs and games and such. I had my data backed up on an external hard drive, and was transferring files back from that onto the internal hard drive. At first, it was going well. A folder a couple hundred of Mb large was transferring in a few seconds.

Suddenly, it slowed to a crawl. A single file of 22 Mb took half an hour to transfer over. And so forth. I have no idea what's causing this sudden slowdown, or how to fix it. Nothing's changed with how I'm doing it. I'm transferring one folder at a time, no single transfer over a couple hundred Mb, from my external to my internal; at first it was going fast and now it's slow as heck.

Why?

Thanks.

(My antivirus is Avira, the Norton was a free trial that came with the computer)

EDIT: Also, I've already tried shutting down my computer and turning off the external HD for a full five minutes, and it didn't help.