I think the card slot is loose on some phones.
Went to storage, made sure card was unmounted.
I powered the phone off, removed battery, removed SD card for about a minute( don't think it has to be a full minute) put it back in ( making sure it was all the way in). Then powered it back on. Worked fine after that.
Might want to check if fastboot in settings is checked. Uncheck it and try.
Might want to unmount card and remount from settings, with fast boot Unchecked, reboot and see if anything changes.
Heard different info as far as fastboot being checked or not, so if it doesn't Change, try it with fastboot checked.
I did it with it Unchecked, but you may have better results with it checked.
Hope it works out for you