virtualbox.org

Trying to install Windows NT 3.51 Server with the latest virtualbox for windows (running windows 7). The cd-media as .iso which is added to the IDE controller, primary ide-slave does not seem to be bootable like nt 4 media. Nevertheless, i have 3 bootdisks as .img files for the 3.51 installation. But in virtualbox only 2 bootdisks can be added to the floppydisk controller.... However the installation does start up but stops when the 3rd bootdisk is needed.. I am stuck at this point - how to proceed?

You only mount one floppy image, i.e. disk 1. The installer will prompt when its ready for you to insert the next floppy in the same drive.

Remove the second floppy drive from the VM recipe.