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512K gives System Error = 02 reading floppy / HD20


Active member
I’ve got an unmodified 512K logic board that boots up fine, but when I double-click (or use File > Open) on an internal or external 400k floppy, it gives me “the bomb” with System Error ID = 02.

Seems odd that it can read a disk to boot, but can’t view the disk content from the desktop.
It will even boot to an HD 20 external drive (using a 400k floppy with the HD20 Startup extension), but gives the same 02 System Error when I double-click the HD 20 icon.

It’s not the drive or the floppy cable; I’ve tried alternates with the same result.

Further oddity: I do not get the error if I use a 400k boot disk for an original 128k Mac!
System Error ID = 02 is an Address Error.

Anyone have any idea what on the board would cause this? Processor? IC chip? Capacitor? (C5 looks slightly distressed on one end.)
Thanks for reading!


Well-known member
Are the floppies formatted as MFS (Macintosh File System)? Are they actually formatted as 400k, and not 800k?

An unmodified 512k with original ROMs can't deal properly with the newer HFS filesystem that newer Macs use, but booting form the HD20 floppy should take care of that because it enables HFS support by loading an INIT that patches the ROM. Hmm....



Active member
Thanks for the suggestions! The floppies I’m using are absolutely 400k; and I’m fairly certain they’re MFS.

But the Mac 512K boots fine from these floppies So they’d have to be 400K MFS, wouldn’t they? If they weren’t, I’d get the Sad Mac and 0F0064 or whatever, right?

After booting, though, it can’t open a window on the desktop to display the disk’s content. That’s when the system crashes with the 02 error.

I’m wondering whether this error means a chip is defective; if so, which? IWM? VIA?

Or could it be an electrical problem caused by the ragged-looking capacitor at C5?


Active member
For the record: I found the problem!
The desktop folder on the HD20 was infected with CDEF, a “classic” computer virus.
Evidently CDEF, like it’s predecessor MDEF, can cause 128K & 512K macs to crash.
Disinfectant 2.1 found it. It also found several instances of nVir.
This discovery prompted me to run Virex and Disinfectant on my 2 GB software archive. 20 infected files were found!
Surprising how widespread it was. 32 years late, these vintage Mac zombies nearly ate my brain!
So happy to have my 512K/HD20 rig working!


Well-known member
Heh, that's funny. A couple of years ago I was going through a big stack of old floppies after I resurrected my Plus.
Disinfectant flagged one of them with MDEF. i though it was pretty funny, 30 year old virus still lurking.
Back in the day these things were pretty common around the office with everyone passing floppies around.


Active member
I let my guard down, thinking old Mac viruses were no threat anymore. I understand these old viruses weren’t terribly malicious, as viruses go, on the Mac II-era systems they were targeting. This obscure effect on older systems like my 512k was way more annoying.


Well-known member
The good thing is that today, unlike then, there should be no meaningful damage (you are unlikely to do your accounting on a 512k, for example).


Active member
I spoke too broadly. CDEF, which I just encountered, seems to have been less troublesome at the time than MDEF & other viruses — according to lowendmac, anyway.