Using an "SCA to 50-pin adapter", it is possible to use a 300 gig Ultra-320 SCSI drive from 2004, with an Apple II outfitted with the Apple SCSI controller.

It seems possible for the electronics to be capable of accessing more storage than what the available software is currently designed to support. So...

Looking at the electronics only, what is the theoretical maximum block number / disk capacity accessible by the old SCSI controller?

If the old controller has technical limits on the maximum accessible block number, is there a way for the drive to overcome those limits?

Software-wise, what is the maximum usable logical volume or number of volumes that can be created?

  • 2
    I think SCSI is completely backwards compatible, so in theory you should be able to connect the earliest SCSI drive to the latest SCSI controller and vice-versa.
    – bjb
    Commented Mar 13, 2017 at 20:54
  • 1
    @bjb this is almost correct, you should ask your own question for that if it hasn't been answered. Commented Mar 14, 2017 at 2:29
  • @Dale Mahalko do you have any documentation on the scsi interface? I was only aware of the mfm drive. I think the largest drive I saw advertised was 40mb, but that was from before prodos. Commented Mar 14, 2017 at 2:43
  • @bjb: Except maybe for SASI drives. Also, there were SCSI versions. Commented Mar 14, 2017 at 20:46
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    Also, if you're including the IIgs, the maximum partition size is 2GB with HFS. Commented Mar 14, 2017 at 21:09

1 Answer 1


The SCSI controller is not a limiting factor. The Apple II SCSI controller electronics, and SCSI protocol, would support accessing the blocks of a very large disk. However, managing those blocks is all done by some DOS software, and ProDOS imposes some more severe constraints.

ProDOS manages the disk as volumes and partitions, and does both these in a manner appropriate for an 8-bit, 64 KB computer - sparingly. Volumes are associated with slots, and partitions are kept small (by modern standards) to minimize the file system management overhead.

For ProDOS 2.0 and above, the maximum partition size is 32 MB and the maximum number of partitions is, theoretically, 13. To get to 13 you have to minimize other cards in the system and remove Floppy Drive Controllers. 32M x 13 would be a maximum of 416 MB for ProDOS HD volumes.

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    Of course, back in those days, 416 MB was a massive amount of storage. I think people don't quite appreciate how much a megabyte really was back then...
    – user
    Commented May 14, 2017 at 11:58
  • Especially given lack of any real networking... I had a want to say 300 MB hard drive on my Apple IIGS (partitioned basically as you said ... ten folders sounds right) and basically every month would add 1-2 800k floppies (softdisk) and there was basically no way I was going to ever fill it .... then I went off to college with a 540 MB hard drive on a 486 with 10 Mbps ethernet and I think I filled that up about a month in or less
    – Foon
    Commented Oct 27, 2020 at 20:52

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