7.6) How do I modify Atari DOS to support more than two drives?

Subject: 7.6) How do I modify Atari DOS to support more than two drives?

Russ Gilbert writes:

DOS 2 type DOSes default to two drives.
Boot DOS and BASIC.
? PEEK(1802)
if it is 3 then you only recognize two drives.
To get the 3rd drive, poke 1802,7. Then go to dup.sys (type DOS)
and write system files. Next time you boot that DOS, you'll recognize
3 drives.
If you want four drives, poke 1802,15.