It worked.  And now I know what the markings on my original boot disk
mean
A=5, B=8 C=D

Darmuk his eyes open.

So, just for anyone who reads this post in the future, I first made a
backup of the original disk and created a new boot disk that expects 5
1/4" for both A and B.

After booting to RDOS and at the ; prompt:

(assume Return at the end of every line)

A;;;
S0 (that's s0 "zero" not sO)
RD100 S7E00 1
[INSERT BLANK FORMATTED DISK "BACKUP DISK"]
S0
WD100 S7E00 1
[INSERT ORIG DISK]
SE
RD100 S7E00 1
[INSERT BACKUP DISK]
SE
WD100 S7E00 1
[INSERT ORIG DISK]
S1C
RD100 S6C00 1
[INSERT BACKUP DISK]
S1C
WD100 S6C00 1
S0
B  (to boot CDOS to new disk)

then I ran CDOSGEN.

Bill