I created a virtual hard disk with vmware and also with virtual box. I booted from a virtual disk, ran fdisk 1.31, created the partition, looked, it was set to active partition, so everything seemed to be okay. At the end of fdisk I got an error message:
"error reading hard disk. Function number or drive not permitted. Program terminated". Then:
"A:\Press Esc to exit fdisk" (including "A:\". After ENTER I got "A:\"
After a reboot and running format c: and sys c: the virtual HD does not boot.
I tried the same with M$ fdisk and FD format / sys c: - and the (new created) virtual HD boots at once.
I once again tested it with FD fdisk and FD format / sys - it did not boot.
After an ADDITIONAL fdisk /mbr it works too. So it seems that there is a problem with writing the mbr when running from the fdisk GUI.