I formatted my 200 GB Sandisk card in laptop (Ubuntu 16.04) to ext4. No permission problems. Might be that ubuntu automatically considered that the card should be usable by anyone and wrote access rights accordingly.

My 128 GB card seems to be completely broken, I was not able recover any partition info via GParted nor testdisk and ŕeformatting it failed both using the phone and laptop. I guess I should try to do some post mortem analysis, but cannot get bothered with it. I had backups of most of the content so not a big deal for me. Sad thing is that the 128 GB Sandisk card lasted under 3 years. Even sadder thing is that Jolla phone did not warn me about non accessible sdcard. I noticed problems when listening stored music stopped working.

I have 64 GB Sandisk card in N900, it is possibly a month younger than the phone (I got mine on December 2009).