On Wed, 17 Mar 2021 10:08:56 +0100
There were some conversations on this on alt.os.development a few years
back.
Specifically, I never could get USB-FDD to work correctly for MS-DOS.
USB-HDD eventually worked, but not for the machine I wanted. This
implied that the BIOS emulation on one of the machines was flawed.
From a post of mine to a.o.d. in 2017, there was some more info on each
of the three modes:
"
USB-FDD is DL=0, no MBR, no partition, apparently needs a correct
device name, e.g., manufacturer's name of a physical floppy device
USB-ZIP is DL=0, one partition, boots VBR directly, skips boot code, 63
head/32 sectors preferred, translates disk calls from start of the
partition which hides data prior to the partition start, may need to
modify values in the VBR to boot correctly
USB-HDD is DL=80h, two partitions, or it may auto-select USB-ZIP with
only one partition for some BIOSes
"
Apparently, I found a clue indicating that there is a specification for
the modes out there somewhere.
--
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