Cannot write to usb drive with FAT partition

I take out a USB drive with FAT partition. Gnome Files can mount without issues. Copy files out OK.

When trying to write something to the drive, Gnome Files just give no response, and file not copied.

When using terminal, I got:

cp: cannot create regular file '/run/media/fcc/openwrt-ipq807x-generic-dynalink_dl-wrx36-initramfs-uImage.itb': Permission denied

What should I check to find out why there is a permission issue?

This may be a sign of a corrupted filesystem on the disk.
Try using fsck.fat from the package dosfstools.

fsck reported FAT mismatch and Dirty bit is set.

After copy Original to Backup and clear Dirty bit, I can write files to the USB drive now.

Thank you very much!