Bootable FAI installer USB stick without ISOHybrid?

[Alexander Thomas]

Another episode in the saga of bootable USB sticks that are also
usable in Windows.

In short: would it be possible to make a FAI installer boot and run
from a USB disk that has a normal MBR partition table, instead of the
ISOHybrid hocus-pocus?

The long story:
We haven't quite given up yet on the idea of a single USB stick that
contains both a FAI installer and also a partition that is easily
accessible and writeable under Windows. It must also be possible to
add or at least expand that partition using Windows tools without
breaking the ability to boot from it.

Windows used to only support a single partition on removable drives,
but they changed this with Win10 build 1703. It supports multiple
partitions on removable USB drives, but as can be seen from my
previous messages, this does not work with an ISOHybrid drive. It
seems the hocus-pocus ISOHybrid uses to create a disk that is bootable
from both a CD-ROM and a USB stick, is just too weird for Windows.
Different versions show different behaviour, but in a recent Win10
only the EFI partition is shown and the partitioning tool wants to
nuke the entire disk. GParted also doesn't know what to do with such a
disk.

We don't need the ability to boot from a CD-ROM drive and we don't
need to support all kinds of platforms like Macs, we only care about
booting a typical modern PC architecture server from USB and running a
standalone FAI installer from it. Although legacy boot support with
the same disk image would be nice, it would be OK if it only works
with UEFI.

So the question here is what it would take to run the FAI installer
from a USB stick that has a regular MBR partition table. Booting a
Linux system from a USB drive shouldn't be hard, I can vaguely
remember having done it quite long ago. The biggest questions are
probably how to do it with minimal changes, and also whether there's
anything inside FAI that assumes an ISOHybrid environment and would
break when booted differently.

Regards,
Alexander