I'm trying to boot a custom made-in-ASM OS on my recent laptop. The OS is intended to be installed on a floppy and during make creates a bootable floppy.

Since I don't have a floppy drive, I installed it on a virtual floppy. After that I used WinToFlash's create bootable MS-DOS USB drive option to transfer the floppy image to an USB flash drive. Then I tried to boot my computer from it but got only a repeating broken string on screen.

After all that I made a virtual hard disk image form the flash drive using this tutorial and tried to boot a virtual machine from it. First time I got same problem as on real computer. I then used the reset option and next time and every time after that OS booted correctly.

My question is: How do I figure out what exactly happened to the virtual machine between first and second boot?

UPDATE I just created a new VM with default settings for windows XP and it has the same problem that I have on a real computer. I was unable to reproduce the procedure which made the first VM work correctly.