I make more test with floppy BOOT-PROM3 and floppy image over PXE server and grub4dos from USB stick. With vendor bios floppy or floppy image boot OK from all: FDD, image over PXE and image over grub4dos from USB stick.
With coreboot + SeaBIOS is very interesting situation: boot OK only image over PXE server with gPXE
Not boot from real FDD or image from USB with grub4dos. While boot from USB have this info: ========================================================= Autodetect number-of-heads failed. Use default value 2 Autodetect sectors-per-track failed. Use default value 18 ========================================================= and after some seconds, menu appears and next is same problem like from real FDD
BOOT-PROM3 floppy image is non standard and while try open it e.g. in WinHex have warning http://savepic.net/4142258.htm that sectors count is less than in boot sector but why this image or floppy working only with vendor BIOS or from PXE server? Why not boot over grub4dos from USB and real FDD only from coreboot bios?
Link for BOOT-PROM3 floppy image: http://www49.zippyshare.com/v/75868779/file.html Please test this image with PXE, grub4dos from USB and real floppy in QEMU.