it happened again. i don’t know what is the cause but it happened. The file or directory in my flash usb can not be deleted. I m curious so trying to check a few thing like dmesg command. To see what messege appeared when plugging the flash.
Bingo, there was some error. So I unplug the flash and plug it in one more time. Check by dmesg again, and no error.
Now I can mv, cp or anything to my file in the flash usb.