When running Atari DOS II and compatibles, memory location 1802 ($70A,
DRVBYT) indicates the number of disk drives allocated. At the Atari BASIC
READY prompt, enter "? PEEK(1802)" to read the value of this location.
Possible values include:
1 = Drive 1 only
3 = Drives 1 and 2 (default value)
7 = Drives 1, 2, and 3
15 = Drives 1, 2, 3, and 4
The value of DRVBYT can be changed with the Atari BASIC POKE command. For
example, "POKE 1802,7" to set DOS to support drives 1-3.
To save a changed value for DRVBYT that will be in effect when the computer
starts up, go to the DOS menu (enter "DOS" at the READY prompt), then choose
menu option H, Write DOS Files. This disk will now boot with support for the
number of disk drives of your choosing.