It works. The HD20 can be used with MFS. If only developers had know this back in 1985, it would have solved many problems with applications that took many months to become HFS compatible.
As of this writing, I've only successfully formatted the drive MFS using the stock System 2.0/Finder 4.1 on a 512Ke which obviously has the HD20 drivers built-in along with the 800K driver and HFS. Since the system doesn't know how to implement HFS, erasing the drive produces a 20MB MFS volume, which with limited testing seems to function perfectly. To double check this, I then booted with an HFS system which showed the appropriate pixel on the left-hand side of the System volume windows and no pixel on the HD 20 volume windows. Rebuilding the HD 20 desktop and also deleted any "virtual" folders. Using this drive with an installed system on a stock 64K ROM 512K with the standard HD20 startup disk, boots up normally, spitting the HD20 disk out and the drive running under its own system. If an HFS system is installed on the drive, after it loads the Mac it likewise reports the HD 20 volume as MFS, while correctly reporting other HFS disks as such. I would NOT run it this way, however.
While the HD 20 startup disk works with the HD 20 on a 512K, I haven't found an MFS system configuration that formats it correctly and I'm not sure why. Basically, it formats it with significantly less space (5MB less). Depending on the system, HFS, MFS or version that is used, it reports everything from 0K available, to 30MB. I suspect this is the same issue I experienced with using the same 800K floppy to format MFS & HFS in earlier tests. Some of those disks ended up showing 1.4MB available. I'll report back when I have time to do a low level format of the drive and then do a clean format from one of the "Magic" Systems discussed above.
Now I know JDW has indicated that there is little need for a 20MB MFS volume because of the organizational problems since all files reside on the root directory only and can't be truly nested directory style. I don't necessarily disagree. There is also another issue with the MFS Finder 4.1 which restricts the number of files on any one volume to 128 to 500 depending on the Mac. So unless you have some really big files, you won't even be able to use all 20MB. Also, the Finder responds more slowly the more files it has to manage. Ultimately you'll end up with a very sluggish system on an under utilized drive. The solution prior to HFS was to use disk partitioning, which also limited you to being able to mount a limited number of partition volumes at once. Unfortunately, this is a hack Apple never intended anyone to find out about and thus there is no partitioning software that I know of for this express purpose. I'll be posting some details about MacServe eventually which will partition an HD 20 with some limitations, but MacServe was designed to be an AppleTalk disk server, so it eats up a lot of RAM in the process of simply partitioning your hard disk.
And then there's the fact that the 64K ROM Mac is inherently an MFS environment which is best served by not using up your valuable RAM with HFS code. Unfortunately, the 128K has been restricted by Apple from ever loading the 800K disk drivers and HD 20, which would only take up a minor piece of the limited RAM (certainly no more than the hard drive drivers that were marketed for the 128K at the time).
Nevertheless, having used the HD 20 which is certainly slow on even a Mac Plus, I have decided that a 20MB flat filing system is preferable to the HFS nested one. Why? If you know the contents of your 20MB disk intimately, it's easy to go straight to the folder and find what you need. But heaven help you if you forget where you put something as I do all the time. Even an HD 20 with a dozen folders, each with one or two sub-folders can take what seems like an eternity to open and search all of them. As you know, there is no search function built into the old finder and you are limited as to how you can view the files on a disk. With a flat filing system under MFS, this problem goes away. You may have an enormous list of files all on the root directory, but at least you can sort them alphabetically, date, size and kind to quickly find what you are looking for. I would actually be willing to bet money on this that I could find a file on an MFS-based HD 20 faster than someone using the HFS version who has to dig through folders.
There's more to research here, but obviously it's a great alternative to have.
As of this writing, I've only successfully formatted the drive MFS using the stock System 2.0/Finder 4.1 on a 512Ke which obviously has the HD20 drivers built-in along with the 800K driver and HFS. Since the system doesn't know how to implement HFS, erasing the drive produces a 20MB MFS volume, which with limited testing seems to function perfectly. To double check this, I then booted with an HFS system which showed the appropriate pixel on the left-hand side of the System volume windows and no pixel on the HD 20 volume windows. Rebuilding the HD 20 desktop and also deleted any "virtual" folders. Using this drive with an installed system on a stock 64K ROM 512K with the standard HD20 startup disk, boots up normally, spitting the HD20 disk out and the drive running under its own system. If an HFS system is installed on the drive, after it loads the Mac it likewise reports the HD 20 volume as MFS, while correctly reporting other HFS disks as such. I would NOT run it this way, however.
While the HD 20 startup disk works with the HD 20 on a 512K, I haven't found an MFS system configuration that formats it correctly and I'm not sure why. Basically, it formats it with significantly less space (5MB less). Depending on the system, HFS, MFS or version that is used, it reports everything from 0K available, to 30MB. I suspect this is the same issue I experienced with using the same 800K floppy to format MFS & HFS in earlier tests. Some of those disks ended up showing 1.4MB available. I'll report back when I have time to do a low level format of the drive and then do a clean format from one of the "Magic" Systems discussed above.
Now I know JDW has indicated that there is little need for a 20MB MFS volume because of the organizational problems since all files reside on the root directory only and can't be truly nested directory style. I don't necessarily disagree. There is also another issue with the MFS Finder 4.1 which restricts the number of files on any one volume to 128 to 500 depending on the Mac. So unless you have some really big files, you won't even be able to use all 20MB. Also, the Finder responds more slowly the more files it has to manage. Ultimately you'll end up with a very sluggish system on an under utilized drive. The solution prior to HFS was to use disk partitioning, which also limited you to being able to mount a limited number of partition volumes at once. Unfortunately, this is a hack Apple never intended anyone to find out about and thus there is no partitioning software that I know of for this express purpose. I'll be posting some details about MacServe eventually which will partition an HD 20 with some limitations, but MacServe was designed to be an AppleTalk disk server, so it eats up a lot of RAM in the process of simply partitioning your hard disk.
And then there's the fact that the 64K ROM Mac is inherently an MFS environment which is best served by not using up your valuable RAM with HFS code. Unfortunately, the 128K has been restricted by Apple from ever loading the 800K disk drivers and HD 20, which would only take up a minor piece of the limited RAM (certainly no more than the hard drive drivers that were marketed for the 128K at the time).
Nevertheless, having used the HD 20 which is certainly slow on even a Mac Plus, I have decided that a 20MB flat filing system is preferable to the HFS nested one. Why? If you know the contents of your 20MB disk intimately, it's easy to go straight to the folder and find what you need. But heaven help you if you forget where you put something as I do all the time. Even an HD 20 with a dozen folders, each with one or two sub-folders can take what seems like an eternity to open and search all of them. As you know, there is no search function built into the old finder and you are limited as to how you can view the files on a disk. With a flat filing system under MFS, this problem goes away. You may have an enormous list of files all on the root directory, but at least you can sort them alphabetically, date, size and kind to quickly find what you are looking for. I would actually be willing to bet money on this that I could find a file on an MFS-based HD 20 faster than someone using the HFS version who has to dig through folders.
There's more to research here, but obviously it's a great alternative to have.