There are two physical media sizes, 3½" and 5¼". This document concentrates on 3½" drives and media, since 3½" MO drives are cheaper than 5¼" ones, and I have only had experience with this type of drive and media. Please contact me if you have a 5¼" MO drive.
A 3½" MO disk looks similar to a 3½" floppy disk, except it is twice as thick. There is a metal shutter covering the disk surface, and a write-protect tab in the corner, just like a floppy disk. If you open the shutter, the disk inside has a similar appearance to the surface of a CD.
Whilst there are two physical disk sizes, there are several different disk capacities. 3½" MO disks are currently available in the following capacities:
5¼" MO disks are currently available in the following capacities:
The only difference between 540MB and 640MB MO disks is the sector size. It is 512 bytes for 128MB, 230MB and 540MB disks, but 2048 bytes for 640MB disks. There is no real advantage to Amiga users in buying 540MB disks if you have a 640MB drive, unless you for some reason want to use an FFS version earlier than 40.1 (see the Amiga Requirements section), or use an operating system which does not support sector sizes other than 512 bytes, such as NetBSD. Also performance of 512-byte-sectors media with Macintosh emulators may be better. A similar thing applies to 5¼" MO disks, 600MB vs 650MB, 4.1GB vs 5.2GB, etc.
3½" MO disks are single-sided. 5¼" ones are double-sided, and half the quoted capacity is accessed at a time. The disk is turned over to access the other half.
MO media can be bought as unformatted, or formatted for MS-DOS or Macintosh. If you're using a Mac emulator, you may wish to buy Mac formatted disks. For Amiga use it doesn't matter; you can just do a Format QUICK on the disk to make it ready for use, since the disks are low-level formatted at the factory.
MO disks are low-level formatted and usually certified at the factory. Certified disks have had their surface checked and any bad sectors are remapped. If you buy an uncertified disk (I don't know whether these are available), it will be a good idea to low-level format it before use.
More recent disk capacities (230MB, 540MB and 640MB for 3½" disks) are also available in LIMDOW variants, also known as "direct overwrite". Writing to LIMDOW disks is faster than with normal disks.
In common with hard disk manufacturers, MO disk capacities are usually quoted in millions of bytes, as opposed to the usual computer terminology where 1 megabyte = 1024×1024 = 1,048,576 bytes. The quoted figure is the unformatted capacity. For example, the formatted capacity of a 640MB 3½" disk is 635,600,896 bytes, which is just over 606 megabytes.