Hard drive light stays on

I just installed a DVD+-R/W drive and a couple old hard drives alongside my
SATA hard drive. Everything seems to be working great, except one of the
hard drives which wouldn't show up, so I took it out. Both before and after
I took that hard drive out, though, the red hard drive light on the front of
the computer case has remained lit. I unplugged the other old hard drive and
its IDE cable and it's still lit. The new DVD drive is functioning fine.

Since everything is working, I'm at a loss as to why this hard drive light
is staying lit.

Jon