List of Famicom Disk System error messages
Err. 01 = No disk card.
Err. 02 = No disk power supply — batteries and/or AC adaptor.
Err. 03 = Disk card is write protected.
Err. 04 = Disk card not authenticated (game maker ID).
Err. 05 = Disk card not authenticated (game name ID).
Err. 06 = Disk card not compatible (version ID).
Err. 07 = Wrong side of disk card set in drive.
Err. 08 = Disk card #1 wrong.
Err. 09 = Disk card #2 wrong.
Err. 10 = Disk card #3 wrong.
Err. 20 = Screen data wrong.
Err. 21 = Disk card header block (Nintendo-HVC) wrong.
Err. 22 = Disk card header block #$01 unrecognized.
Err. 23 = Disk card header block #$02 unrecognized.
Err. 24 = Disk card header block #$03 unrecognized.
Err. 25 = Disk card header block #$04 unrecognized.
Err. 26 = Unable to write to disk card.
Err. 27 = Block end mark seen but ends prematurely.
Err. 28 = File ends prematurely during read.
Err. 29 = File ends prematurely during write.
Err. 30 = No space left on disk card (full).
Err. 31 = File count in header and number of files on disk card do not match.
(sourced from Famicom World forum www.famicomworld.com)