Suddenly lost permissions on SD card

I have been using a micro SD card, via a card adapter, for taking music from the mac to a music production platform. But suddenly, I lost permissions to write to the SD card.

There is no lock on the card to allow me to change the permissions.

I tried repairing the disk, but cannot find anything on resetting permissions, other than instructions on how to reinstall the OS.

What else can I try - on Big Sur - short of a complete reinstall?

I'm sure there used to be a really simple way to reset permissions, but it is apparently gone from disk utility. If anyone understand why, I would be interested to know.

Thanks.