I took an Osborne 1b ROM rev 1.4 to the 2600 magazine "The Last HOPE" convention on July 19th, as part of a vintage computer demo. I originally planned to exhibit an "original" version of the Osborne 1 (with the tan chassis, also model rev v. 1.4), but that one had a bad A drive. "Insert Disk in drive A and press RETURN" is as far as I could get.
Prior to the event I did attempt to replace the drive. First I had to see what was going on inside of the machine. It takes about 10 minutes to completely disassemble an Osborne 1, if you have a longish phillips-head screwdriver. There are only three types of screws and clearly this machine was made for field repairs.
Anyway.. once I got the system completely apart so that I could get to the disk drives I learned that the drive gets it's 5v and 12v power from the data cable, and there is no separate power connector like your run-of-the-mill IBM PC drive. The drive's controller card had special electronics for drawing voltage through certain pins in the dataline, etc.
In short, one cannot simply swap out an Osborne 1 disk drive and replace with the drive from a Kaypro II or IBM PC. My choices appeared to be either sacrifice a working drive and swap the electronics from the bad Osborne drive, or simply find a replacement drive.
It was 11 PM and I decided to take the Model 1b instead.
Now here is why all of that trouble was worth it. the next day at the exhibit, I ran into a guy who used to be an Osborne service tech. I got into discussing the system I left at home with the bad A drive. He told me that a special key combination, when pressed upon boot, would cause the system to boot from the "B" drive. CP/M needs to boot from A, so A becomes B, B becomes A when this bypass is activated. He tried for quite a while, but could not find the key combo.
After he left I found the answer on http://vintage-computer.com/vcform/
...when pressed upon boot will cause the system to swap the drives.
The B drive (now "A") DOES boot, and now I can use the system with one drive at least.
Anyone have a spare Osborne disk drive preferably jumpered to A?