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Format 800K MFS with 64K ROMs

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FYI, this seems to be a very odd entry in the Apple Knowledge Base:




and seems to suggest something that is not supposed to be possible. It makes me wonder if an HD20 could be formatted on a Mac Plus with System 2.0/4.1, basically resulting in a 20MB MFS volume that could then be used with a 64K ROM Mac.


Otherwise, why have a tech note about it? I will test and report back.

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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.

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...the Finder responds more slowly the more files it has to manage.

That is certainly true, even on an HFS formatted drive like my HD20. I have quite a large number of files stored on it, and it takes a LONG time to display a window in the Finder for the first time after cold boot.



Mac128, many thanks for your outstanding report. You clearly spent considerable time investigating this AND in typing it in for all to read! Hopefully the owner's of this 68kMLA forum are regularly backing up server data (and then confirming the backups are good) so we don't lose all this important information ever again!


I personally look forward to hearing more about your successes in partitioning the HD20, with some MFS and some HFS partitions and how well they interact with each other. Certainly, if the software you have takes up too much RAM to do its job, then it's not a practical solution. In such an event, I can only hope that the Nested Volume Manager would be an alternative partitioner. But it would take a programmer among us to compile the code given in that MacTech article. It looks like all the code is there in that article, but I myself am not sure how to piece it all together and compile it into a standalone app.


Anyway, once again, thank you Mac128 for your dedication to the classic Mac community. No doubt we have many other classic Mac lovers out there who actively tinker with their machines in technical ways; but sadly, only a select few "white knights" like Mac128 make time to sit down and report about those effects. Kudos, Mac128!

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In order to format the Hard Disk 20 using a stock Mac 512K with 64K ROMs, you'll need to use the "Magic" System 2.0/4.1 as described above, with the HD20 INIT and erase the HD20 as usual. This will reformat the disk as MFS. But here's the catch, you can't actually use this or any MFS only system with an MFS formatted HD20 with 64K ROMs, because Finder 4.1 and earlier doesn't know how to handle a 20MB volume, even though it has no problem formatting it. On a 512Ke, the information the Finder needs to handle that same 20MB is already in the ROM, so Finder 4.1 and earlier work just fine in decoding it. Evidently the RAM-based HFS leaves a few things out.


So, in order to use this spacious 20MB MFS disk with 64K ROMs, you'll need to use an HFS aware system, which will treat the HD20 the same way it treats a normal 400K disk, as MFS, reading and writing MFS. That means Finder 5.0 or above. You just won't be able to format the HD20 as MFS using these systems, they will only format HFS. Then again, you only need to format it once.


In a quick test I discovered that MacServe will happily format an HD20 MFS formatted disk using its own partition map. Once the HD20 has been formatted with MacServe, you can use any System you like, including System 2.0/Finder 4.1. Evidently MacServe DOES know how to handle 20MB of MFS storage and lets the earlier Finder know how to deal with it. Whatever the case it works.


One further note about MacServe as I continue to experiment with it, using an HFS aware System with MacServe will also let you format the HD20 as MFS and then sub-partition the MFS volume with both HFS and MFS volume partitions. Using Finder 4.1 or earlier, will only let you partition one way, MFS. I haven't had the time really to work with anything much, but this configuration seems solid. I'll write this all up eventually and put it out there for others to experiment with as well.

Edited by Guest

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Hate tro bring up this old post, but I'm going to try and get that nested volume manager together for you :)


EDIT: on a further note, This may be an issue because I can't seem to find a copy of Mega-whatever C that he uses, and the code is.. incomplete in some places. I'll look at it further when my eyes arent so blurry.

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I have now tested the following 3.5" External Drives with a 64K ROMs:


Apple UniDisk 3.5" – MFD-51W mechanism

Introduced in September 1985 with the HD20

This drive works with the Liron interface bypassed without the HD20 INIT.

This drive interestingly will not work with the 800K Ext. Drive cable (with or without the HD20 INIT).

However, it will work with the 400K Ext. Drive cable as well without the HD20 INIT.


Macintosh 800K External Drive – MFD-51W-10 mechanism

Introduced January 1986 with the Mac Plus

This drive does not work with any cable without the HD20 INIT.


Apple 3.5" Drive – MFD-51W-03 mechanism

Introduced September 1986 with the IIGS

This drive works with any cable but the 800K Ext. Drive cable and the UniDisk cable causes continuos eject, without the HD20 INIT.

(The FDHD Superdrive MFD-75W-01G also works like an 800K Apple 3.5" Drive as well)


What's interesting here is that the UniDisk for the Apple II was released well before the 800K Drive was available for the Mac and could be used with or without the HD20 INIT with 64K ROMs, but the internal Liron interface card disabled it from otherwise working with the Mac. The newer 800K Ext. Drive did not have an interface card, so it seems Apple deliberately disabled an updated version of the Sony drive (-10) and changed something about the DB-19 cable to further disable it, in case the UniDisk drive-type mechanism (which is otherwise compatible) ended up in it, so it would still only function with the HD20 INIT. I'm sure in part, this kept anyone from attempting to continue to use MFS with the 800K drives even though it is easily possible -- particularly where the 128K was concerned which could not use HFS. What better reason to upgrade to a Mac Plus?

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