I've "read" that the HD20 cannot boot a Mac 128k, and it theoretically makes sense why, but I've yet to read an article say: "Yes, I've tested and confirmed that the HD20 will NOT boot a Mac128k with or without the HD20 init, with or without 128k ROMs." The reason I mention this is because I've seen quite a lot of mistakes reported on the web about what will and will not work on old Macs. The best thing to do is test it yourself, I say. And to do that properly, you would need to use a variety of different OS versions, because some may work while others not. I've found this to be true of getting the HD20 to work on my Mac512. I can run up to Finder version 6.0 on it (using a System 4.x or 5.x), but 6.0.5 or 6.0.8 won't boot at all.
I don't know about the protocol used between the HD20 and the Mac. But since you mention MFS, perhaps this
ADC TechNote may be of interest.
Since MFS supports drives up to 20 megabytes in size and a maximum of 1,400 files, it theoretically should work on an HD20. The reason why Apple prevents you from formatting the HD20 with MFS by default is due to the fact that
MFS is a flat file system. Folders are nothing more than an illusion, thus making file management a real mess even on a drive as small as the HD20. So imagine an HD20 with a thousand files on it, and imagine you have those files organized into MFS's 'fake' folders. When you pull up the Open/Save dialog in a given app, ALL your files will appear in a single list (no folders)! But with HFS, you will see folders in the Open/Save dialog.
Nevertheless, there are times you may want MFS on an HD20 for compatibility reasons. In this case, paritioning the drive and formatting one partition as MFS makes the most sense. This
Nested Volume Manager article has some interesting details on how one may wish to go about doing that.
It is also possible to format 400k floppies as HFS and 800k disks as MFS using some interesting tricks. Here is the trick to format
400k disks as HFS:
* Boot with a disk that has Finder 5.x or 6.x.
* Format/Initialize the 400k floppy while holding down the Option key and clicking One-Sided or Initialize.
Or if the disk is in a 400K disk drive, hold the
Option key down while you press the Return
key after you've named it.
* If you want to duplicate an HFS 400k disk in the Finder (version 5.2 or higher) to another 400k disk while keep it HFS, you need to hold down the Option key during the process.
I have confirmed this on my Mac512. You must select the disk, then hold down OPTION, and then choose Erase from the Special menu. If your 400k disk is in the internal 400k drive, the dialog that appears will give you an "Initialize" button -- clicking that will give you an HFS 400k disk. And if your 400k disk is in an 800k external drive, the dialog will give you a choice of one-sided or two-sided -- clicking one-sided will create a 400k HFS disk.
I have also confirmed that these HFS 400k disks
will not boot any Mac with 64k ROMs, neither in the 400k internal drive nor an A9M0106 external 800k drive. But that makes logical sense seeing the 64k ROMs cannot understand HFS without the HD20 INIT having been loaded first.
I've also "read" on the net that it is just as easy to make an
800k disk MFS:
* Format/initialize your 800k disk with any System/Finder you like.
* Boot with a disk that has an older Finder, like version 4.1
* Select your formatted 800k disk and choose "Erase" from the Special menu.
* And if you want it to be a boot disk, just use Finder 4.1 and an older System.
However, in my own testing, I have never been able to create an MFS 800k disk on a Mac512 with 64k ROMs! So unless someone can show me the steps to the contrary, I must assume this information is either bogus or it only applies to machines that have 128k ROMs. And it makes logical sense why it wouldn't work on a 64k ROM Mac. Look at the steps... The first step says to format the 800k floppy with any System/Finder. In other words, it will be formatted an HFS 800k disk (clearly, you would have loaded the HD20 INIT in order to do this on your 64k ROM Mac). If you then boot with an older System, even version 4.1, without loading the HD20 INIT, the 800k disk is not recognized and you will be asked to Initialize it! And yes, I've tried the 400k HFS steps on 800k disks -- doesn't work. Even if you load the HD20 INIT and hold down the OPTION key during the 800k floppy 2-sided initialization process, it will not be MFS!
At this point, you are likely wondering how to find out if a disk is MFS or HFS! That's easy. Just open a Finder window on the disk in question. Examine the upper left corner. You will see the square Close Box in the Title Bar, and just under that is the information bar (showing the number of items in the window and amount of disk space taken up). And just beneath that you will see two horizontal lines very close to eachother. In between those two lines, at the far left of the window, you may say a single pixel inside. If you do, the disk is HFS. If no pixel, it's MFS. You can spot that single pixel in each of the 3 windows shown in this
screen shot.
Here are some
HD20 photos, with a little commentary on some of them. I use the HD20 with a Mac512 that has the original 64k ROMs. I also have another Mac512 with a HyperDrive 20 (which is a lot faster than the HD20, by the way). I recently connected my HD20 to the HyperDrive Mac in hopes of easily swapping files from hard drive to hard drive. But for some reason the HD20 wouldn't mount on the HyperDrive Mac. The HyperDrive Mac does have a controller snapped onto the CPU, so perhaps that is the cause of the conflict. Hmmm. Anyway, my HyperDrive is formatted MFS and HFS. The HyperDrive creates partitions (called "drawers"), which makes MFS possible, I think. It's also interesting to note that the boot partition/drawer must be MFS on the HyperDrive, as described in
this article (pull up that web page and search for "MFS" to jump to the right spot).
Here is a comprehensive listing of
HFS & MFS technical notes (mostly from the mid-1980's) that are sure to give you hours of enjoyment!