Just a follow-up note on this . . .
Since reformatting and re-copying the boot disk, I was able to boot the Mac Plus with the floppy disk without a problem. The same disk was used when I booted the Mac SE from the Mac Plus's floppy drive.
I have not tried booting the Mac Plus multiple times with the disk but I am guessing it appears bibilit was correct and it was the SE's apparently defective floppy drive that corrupted something on the floppy disk to cause the error on the Mac Plus.
Haven't done anything as far as cleaning the floppy drive in the Mac Plus, but when I opened the Mac Plus to remove the floppy drive to temporarily install it in the SE, I found the Mac Plus needs a lot more than a floppy drive cleaning!
Since reformatting and re-copying the boot disk, I was able to boot the Mac Plus with the floppy disk without a problem. The same disk was used when I booted the Mac SE from the Mac Plus's floppy drive.
I have not tried booting the Mac Plus multiple times with the disk but I am guessing it appears bibilit was correct and it was the SE's apparently defective floppy drive that corrupted something on the floppy disk to cause the error on the Mac Plus.
Haven't done anything as far as cleaning the floppy drive in the Mac Plus, but when I opened the Mac Plus to remove the floppy drive to temporarily install it in the SE, I found the Mac Plus needs a lot more than a floppy drive cleaning!