NT4 sp6 INVALID BOOT DISKETTE

This server was originally setup with one 4 gb IDE boot drive and a 4 gb scisi drive used for graphic images. A second 2 gb IDE drive was later added as a shared data drive for students. We now want to use the scsi for this purpose as image processing is now done on a seperate computer.

If I now remove the slave IDE, I receive the message INVALID BOOT DISKETTE. INSERT BOOT DISKETTE IN A:\. BIOS is set to boot from IDE 0. If I install any IDE drive as a slave (even with out fdisk) the system again boot fine.

What is happening here and how do I fix it? We want to use the drive in another computer.