Error code

Windows Code 21

WINDOWSCode 21
System Error Code· A POSIX/system errno returned when a system call fails.

What it means

Windows is removing this device. (Code 21).

How to fix it

  1. Close the dialog box and wait a few seconds.
  2. if the problem continues click Restart Computer to restart the system.

Not affiliated with Windows. Answer summarized from the manufacturer's manual linked above; consult that source for the authoritative procedure.