Created attachment 205936 [details] Screenshot grahamperrin@momh167-gjp4-8570p:~ % sudo unetbootin When the menu button is clicked, there's no menu. ---- If (instead) I specify the targetdrive at the command line, then the menu is pre-populated but then clicking OK results in a crash – bug 236972, I guess.
Is the drive formatted as FAT (msdosfs) and mounted? These are the prerequisites for suitable drives. P.S. I like the window decorations on your screenshot, which window manager and theme is that? :-)
A commit references this bug: Author: danfe Date: Sun Jan 26 14:22:23 UTC 2020 New revision: 524176 URL: https://svnweb.freebsd.org/changeset/ports/524176 Log: Explicitly tell users that UNetbootin requires mounted MSDOSFS (FAT) filesystem to work against. PR: 239344 Changes: head/sysutils/unetbootin/pkg-message
No feedback in three months, closing. Feel free to reopen if it still does not work for you after ports r524176.