Thread: Floppy Disk is Not Accessible, Not Formatted, or Not Recognized by Windows

SYMPTOMS

When you access a floppy disk, you get any one error messages given below: A:\ is not accessible. The device is not ready.
-or-
Disk is not formatted The disk in drive A is not formatted. Do you want to format it now?
-or-
STOP: The disk media is not recognized, it may not be formatted. The same disk may work properly with MS-DOS or Windows 95, or after format the disk with Windows 98, Windows Millennium Edition, Windows NT, Windows 2000, Windows XP, or Windows Server 2003.

CAUSE

This difficulty happens on disks that do not have media descriptor byte in BIOS parameter block (BPB) of the boot sector. Some old floppy disks do not have media descriptor byte. The media descriptor shows the type of medium presently in a drive. With MS-DOS and Windows 95, you do not have to set the media descriptor byte. So this problem does not happen with these older operating systems. The media descriptor byte is situated in the BPB of the boot sector at offset 21 (15h) and in the first byte of each FAT on the disk.

RESOLUTION

To solve this difficulty, format the floppy disk with Windows 98, Windows Millennium Edition, Windows NT, Windows 2000, Windows XP, or Windows Server 2003.

WORK AROUND

To work around this difficulty, use a disk sector editor to modify the BPB media descriptor byte to the proper value. For example, you can use the DiskProbe tool to do this on a Windows NT 4.0-based, Windows 2000-based, Windows XP-based, or Windows Server 2003-based PC. DiskProbe (Dskprobe.exe) is built-in with the Windows Support Tools for Windows XP Professional and Windows XP 64-Bit Edition, the Windows 2000 Support Tools, and Windows NT 4.0 Resource Kit Support Tools.

The following table lists the most common media descriptor bytes:
Byte Capacity Media Size and Type
-------------------------------------------------

F0 2.88 MB 3.5-inch, 2-sided, 36-sector
F0 1.44 MB 3.5-inch, 2-sided, 18-sector
F9 720K 3.5-inch, 2-sided, 9-sector
F9 1.2 MB 5.25-inch, 2-sided, 15-sector
FD 360K 5.25-inch, 2-sided, 9-sector
FF 320K 5.25-inch, 2-sided, 8-sector
FC 180K 5.25-inch, 1-sided, 9-sector
FE 160K 5.25-inch, 1-sided, 8-sector
FE 250K 8-inch, 1-sided, single-density
FD 500K 8-inch, 2-sided, single-density
FE 1.2 MB 8-inch, 2-sided, double-density
F8 ----- Hard disk

The BPB media descriptor byte is located in sector 0 of the disk, and looks similar to this:
addr data:
0000 EB 3C 90 4D 53 44 4F 53 35 2E 30 00 02 01 01 00
0010 02 E0 00 40 0B F0 09 00 12 00 02 00 00 00 00 00

xx <---- This byte above XX is the media descriptor
byte and is at offset hexadecimal 21 (15h).