Mounting and Using the Drive and Disks

Using a mount file to mount the drive is the best option if you want to use the MO disk for general data storage, and not for booting from. This gives the maximum amount of free disk space, and disks formatted like this will be easily interchangeable between Amigas.

If you want to boot from an MO disk, you need to create an RDB (Rigid Disk Block) on the disk, just as you would with a SCSI hard drive. You can use HDToolBox or another hard disk partitioning program to do this. Using the RDB method, you can create several partitions on a single MO disk. You can adjust the values for Cylinders, Heads and BlocksPerTrack in HDToolBox if you want, as long as:

Cylinders × Heads × BlocksPerTrack = total number of sectors on disk
To find suitable values for these parameters, refer to the prime factors in the table below. For other types of disk you can use PFactor.

If you reduce the (logical) number of blocks per track, the RDB will take up less space on the disk. Be careful not to make it too small though, in case you decide to add another filesystem in future; the RDB may be too small to hold it.

Hopefully an example will make this clearer. 640MB disks have 310352 blocks. 310352 = 2×2×2×2×7×17×163. So some suitable values for Cylinders, Heads and BlocksPerTrack might be

Cylinders = 17×163 = 2771,  Heads = 2,       BlocksPerTrack = 2×2×2×7 = 56

Cylinders = 2×7×163 = 2282, Heads = 2×2 = 4, BlocksPerTrack = 2×17 = 34

HDToolbox needs to be patched if you want to use your drive in "optical memory device" mode. Also, the values for Cylinders, Heads and BlocksPerTrack that HDToolBox (and perhaps other SCSI partitioning software) uses may be "fake". You should adjust the values so that the whole disk is used, as described in the previous paragraph.

Mount files for PC-formatted disks (AT-HD type) may not use the entire disk space; see the example mount files for PC disks.

Here is a table of the number of usable sectors for each type of 3½" MO disk, and 600MB 5¼" disks:

+---------+-------------+-------------+------------------+
|  Disk   | Sector size |  Number of  | Prime factors of |
|  type   |   (bytes)   |   sectors   | number of sectors|
|---------+-------------+-------------+------------------|
|  128MB  |             |    248,826  |   2×3×113×367    |
|---------+             |-------------+------------------|
|  230MB  |     512     |    446,325  |   3×5×5×11×541   |
|---------+             |-------------+------------------|
|  540MB  |             |  1,041,500  |  2×2×5×5×5×2083  |
|---------+-------------+-------------+------------------|
|  640MB  |   2,048     |    310,352  | 2×2×2×2×7×17×163 |
+---------+-------------+-------------+------------------+
|  600MB  |     512     |    576,999  |   3×3×61×1051    |
+---------+-------------+-------------+------------------+

There are several mount files in the DOSDrivers drawer which is part of the Amiga MO FAQ distribution. You can modify these if you need to create a mount file for other filesystems such as PFS2.

TO CREATE SOME KIND OF STANDARD FOR AMIGA-FORMATTED MO DISKS, ALWAYS USE 2 RESERVED BLOCKS.

The exact values of Cylinders, Surfaces, HighCyl and BlocksPerTrack is not important, but you MUST ensure that:

Surfaces × BlocksPerTrack × (HighCyl+1) = total number of sectors on disk

For 640MB 3½" disks, this means that:

Surfaces × BlocksPerTrack × (HighCyl+1) = 310352

This is to ensure that MO disks are easily interchangeable between different Amigas. If Surfaces × BlocksPerTrack × (HighCyl+1) is less than the number of sectors in the disk you waste disk space, and would need to also send a copy of your mount file when sending an MO disk to someone.

If you use a mount file to mount the drive, you can use MO disks just as you would floppy disks. Ejecting and inserting a new disk should be automatically detected by the Amiga.

If you insert an MO disk with RDB after booting your Amiga, you will need to use a program such as SCSIMounter in order to mount the partition(s) on that disk.

It is easy to format a new MO disk for use. If using a mount file, you can just insert the disk and type a command like:

Format DRIVE MO0: NAME "My MO disk" NOICONS FFS QUICK (Replace MO0: with whatever drive name you have mounted MO disks of that capacity as.) The MO disk will be ready for use in a couple of seconds. It is not necessary to do a full format (i.e., not to use the QUICK option), but you can if you want.

You may wish to perform a low-level format on your MO disks at some point. This is advisable if you buy uncertified disks.