As @AnotherElk has said, it’s a bad idea. This is why the GUI option has been removed - to shield users from creating a situation where they can lose all of their data should their SD card malfunction.
That said, I don’t think it’s a bad thing for this option to be available and findable for those who really want it. Keep it out of the GUI so unsuspecting users don’t set themselves up for disaster, but have a ready set of instructions online for those users who have use cases where the internal storage option makes sense.
I don’t think anyone has written up that guide so far. Someone willing to experiment on their own FP3 would perhaps try something like this (
untested, may break all sorts of things, will definitely erase everything off your SD-card):
adb shell sm list-disks adoptable
And you’ll get a response like
Public: 000_00 mounted XXXX-XXXX
With the numbers and Xes being different for each device. Then run
adb shell sm list-disks adoptable
And it’ll return that public disk again, though it may give you a different set of numbers; these are the numbers you’ll use in the next step.
In the case of my FP2, it returned 179_65 in the first step, but 179_64 in the second. I used 179_64.
To format the card, run
adb shell sm set-force-adoptable true
adb shell sm partition disk:000_00 private 100
adb shell sm set-force-adoptable false
With the 0s being replaced with the numbers of your disk. Then run
adb shell sm list-volumes all
And see if it now has 000_00 listed as ‘private’ rather than ‘public’.
Reboot the phone:
adb reboot
and pray that you haven’t broken your system. I tested it on an FP2 running AOSP 7.1.2 (FPOpen 19.11.2) and it worked. Of course there’s no need for this on the FP2 as the option is available through the GUI, and it isn’t proof that it’ll work on the FP3.