For some reason the Note 9 is very picky. I still have not found one that works. I just purchased an expensive 512 hoping it will work. I will post the results. many people belive it is a Samsung Note 9 software issue which they refuse to correct. Who knows.
I had problems with read/write issues starting from the two updates around February time.
Nothing appeared to have changed, apart from there now being multiple notifications that I’d not got write access to my SD card. There was still stuff in the SD card, but I couldn’t format it, nor add items to it.
Checking it in my PC, it showed that nothing was wrong with it, but after two minutes back in the phone, back came the notifications that it needed write permissions.
WeFix were sent out by Samsung, who still couldn’t find anything wrong with the phone, and about half an hour after they left, back came the notifications.
Backed up the files from the old SD card to my PC, phoned my carrier up to say that it was a problem with the SD card and as Samsung have a one year warranty on SD cards, could I have a new one please? – popped it into the phone and it’s worked fine ever since.
I reckon therefore that it’s a problem with any Android updates that get pushed out and mess up the workings of your phone.