nota valid dos disk means probably its borked (read=unvalidated)
first try to test externa drive with ie. xcopy, if it reads some disk and some not then disks are bad. If you cant read any disk then drive may be faulty, or mainboard (or it can be that escom one which is modified).
you can get a new internal drive with all cables, i will suggest ebay user airey36, he is sometime slow, but he sells 30 items per day,which is not a quick task.
when you have at least one workink floppy drive then you can continue with reformatting CF card and installing workbench.
actually if you have PC with CF card reader, you can do it here under WinUAE emulator.
there are many ways to obtain whdload gaming machine
...of to bed
