Strange booting problem

[Chris Murphy]

Actually, another option is to put the legacy OS into a VM where it
can then inherit some of the features of Linux, including LVM support.
Then you can LVM this external drive instead of partitioning it, and
then make an LV (or two or three or whatever) to use as backing for
the VM, and then those VMs will see each LV as a drive, which you can
partition and format with that legacy OS's tools however you want. Or
use qcow2. Lots of options.