fdisk /? <ENTER>
That was the first process I tried. Unfortunately, even "fdisk /?" loads a program called Free FDISK rather than using the one supplied with FreeDOS.
That's very odd. Even if you select Freedos Fdisk directly
from the UBCD menu instead of selecting UBCD DOS first
and then going to the prompt, you should still have the option to run the command.
If you get into fdisk, then answer "yes" to the fat32 initial question. Once inside fdisk, press <esc> to exit fdisk, and you should return to the prompt.
If you got into UBCD DOS options as I described above, you shouldn't
select "DOS apps" -> Freedos Fdisk. Instead, you select "exit" to get to the prompt, and then run "fdisk /?".
If you somehow got to the prompt and you run "fdisk /?", you must receive the help, not the UI. If that's not happening, then you should double check your ubcd503.iso checksum and the burned CD. This is also valid even if you already solved the problem for this particular HDD.
Something else that seems somehow strange to me is that you mentioned a "free fdisk" that is different
from the Freedos Fdisk program.
Although there are several "fdisk-type" programs included in UBCD 5.0.3, I don't know which specific "fdisk version/program" you are seeing displayed when you run "fdisk /?".