A DOS error message. The full message is
Not ready reading drive x
Abort, Retry, Fail?
This meant the drive door was left open or a diskette was not in the floppy drive. Either insert the floppy or close the drive door (turn lever) and press R.
To switch to another drive after the "Current drive is no longer valid>" message is displayed, press F and the drive letter (or I for DOS prior to 4.0).