Cannot make new folders or run files

I’ve had a MyBook Premium 320gb hard drive for about a year now. Today after getting home and plugging the USB cord into my laptop (Dell Inspiron 1720, vista) I found out I can’t make new folders at all on the hard drive. Whenever I try to make a new folder I get the error “Cannot read from source file or disk.” after anywhere from 10-15 seconds. I can’t open anything but the smallest of files on the hard drive, and even then it takes a while and sometimes doesn’t work. For example, when I try to run a notepad document I get the error “Make sure a disk is in the drive you specified.”

I can copy and paste files both wihin the hard drive and to/from the hard drive (though I have not tried anything very big).

I tried running a chkdsk on the drive and it said there were no errors. I can’t figure out what the problem is, help!