• Updated 2023-07-12: Hello, Guest! Welcome back, and be sure to check out this follow-up post about our outage a week or so ago.

What’s happening to my 400k disks?

I have an original Macintosh that has its memory expanded to 512K. I had a number of 400k disks (Finder 4.1 and Finder 5.1) that work ( read and write files) on this computer.
I also have a Macintosh Plus computer that has its memory expanded to 4MB. When I startup the Mac Plus with one of the 400k disks it works fine. I just opened the disks in the Finder and, in some cases, opened an application MacWrite or MacPaint and previously created document (created on the original Macintosh). I did not modify the file on the Mac Plus. I was just happy to open the files.
Now, when I startup these 400k disks on my original Macintosh they boot just fine and the Finder desktop is shown with the 400k disk icon on the desktop. When I double click on the disk icon I get a bomb message box saying
Sorry, a system error has occurred. ID=02.
There is a Restart button that can be selected to restart the computer. All the disks that had been booted on the Mac Plus give this error message.

Why did this happen? I thought these 400k disks could be used on both the original Macintosh and the Mac Plus interchangeably.

Is there a way to get these 400k disks to work again with my original Macintosh computer?


Active member
I have seen that behavior on a 512K Mac when the desktop file became infected with a vintage virus (CDEF?) which produces this error ONLY on 128K & 512K macs. So you should probably run Virex or Disinfectant on these floppies in your Plus to see if it finds something.


Well-known member
If a floppy disk isn't locked, the Mac will write to it as soon as it's inserted and mounted. It's just a small piece of data that records the fact that it's mounted, and the time. When the disk is ejected, it's written again. This is how the Mac knows if a disk wasn't unmounted cleanly and may need repairs. The trouble may be that the head alignment in your Plus is different enough from your 512K that tracks written by the Plus can't be read by the 512K. I'm not sure this is what's happening... just a guess, but it fits the evidence.


Well-known member
That makes sense but I would not expect the failure mode of a bad disk read to be an address error (#02)?

A virus sounds likely EXCEPT it sounds like the Mac Plus started up from these 400K disks without ever (for example) running an attached SCSI drive. Is that right? If so, it’s a mystery to me.
It was the CDEF virus. I used Disinfectant 3.7.1 to repair the disks. Now the disks work again on the original Macintosh.