> 
> There’s a solution, actually…
> 
> CRON that runs every minute… it executes a file, deletes the file, touches the file, changes it permissions.
>

Absolutely no guarantees on data integrity... Modifying the filesystem can take
an arbitrary amount of time, at which time the user can yank out the USB, which
is something they can do at anytime, etc, etc.

Maybe we need to think "out of the box" here, as I was discussing with a friend
about this offline. Do it like ATMs, where they take your card, do what there
is to do, and return it in the end. Have the USB behind a "hatch" that opens
when the drive is to be attached. Then close it... You get the idea. You open
the hatch to let the user unplug it after you have successfully unmounted. For
safety, you can introduce a forced ejection by the user, should they think that
the process is taking too long, but after they have been warned of the potential
issues.