The Amiga uses specially made diskdrives, you can't just slapp on a PC drive sadly. The A4000 come origianly with HD diskdrive but can opperate with DD diskdrive, you'll find these diskdrives at "your" Amiga dealer.
It sounds to me like you got a dirty diskdrive or the component that feeling if a diskette is precent is damaged/lose soldering. So before you go out and buy a new diskdrive check and clean the one you got.
