• The external disk image method you referenced may have worked four years ago when the post was written but not now. Besides this, Crostini is in the process of migrating to Baguette so it's anybody's guess what may or may not work with the VM until the transition is complete. The best you can do at this point is share the SD card with Linux in the Files app then look for it inside the Linux environment at /mnt/chromeos/removable/name_of_card.