Adding additional hard drive

Question:
I was told I could just hook up a 2nd hard drive, but
didn't have any luck with that and when I hooked up the
orginial hard drive back up to the origianl configuration
and now it won't book up, getting all kinds of different
error messages, lastest is fatal error: isass.exe

I'm not real good at this but I was told that unhooking
the hard drive wouldn't have any effect on the data in it.

Any help would be great
thanks
Scott