I am running some non-app store apps on Umbrel as well, namely a temperature sensor that logs data to a text file.
I used to store this file under /mnt/data/umbrel/gan/
and, during an upgrade, it got deleted.
I know I should have had backups in case something went wrong. But what bothers me is that I lost the data while nothing went wrong.
So the question is - where would it be safe to store data?
Cheers!