Recently I am getting this issue... I've had my USB formatted as MS-DOS FAT as I want it to be compatible with both windows and mac. I had some issues with that (even formatting it to MS DOS FAT in Mac gave me an error) so I went with exFAT. Windows is able to read the USB without issues, but in Mac it's not readable. I can see it in Disk Utility though. So far, running 'First aid' on the disk seemed to help, but now not even that solves the problem now. Why is this happening and how would I solve this ? Thanks!