Every time an update for FP Open rolls up, it overrides the write permissions on my sdcard1, making it read only. I can’t be bothered to go bit-shifting on my phone, editing configuration files by hand, so I do what every standard user does - I use an app for that.
I fix this problem every time with an SD Card Permission Fixer. I guess it shifts whatever bits it needs to make the SD card useable as intended for you.
This is what I use. It works with FP Open Marshmallow. Choose whatever floats your boat. Play Store is chock full of them apps, found nothing in F-Droid.
At least I don’t have to do this a lot, since updates don’t happen very often. The last FP Open security update was two and a half months ago (05.12.2016), so that’s not too much of a hassle.
EDIT 20170313 - Works on FP open 17.02.0