Re: Stack popped reading that procedure....
There's a vital, but missing, bit of information, which is that the machine would seem to have just a single disk drive unit, which consists of two recordable disk volumes on a common drive shaft. Each disk volume can store 5MB of data and has its own, independently addressable set of read/write heads. Only one of the disks is removable.
Lets call the volumes F (fixed) and R (removable), and the pair of removable disks they will be backed up to, B1 and B2.
The backup operation the article describes is the only way you can back up both F and R. This requires the following set of operations:
1) remove R and replace it with B1.
2) copy F to B1. This is the backup copy of F.
3) remove B1 and put it temporarily on a desk.
4) replace R.
5) copy R to F.
6) remove R and replace it with B2.
7) copy F to B2. This is the backup copy of R.
8) remove and store B2 in the data safe.
9) replace B2 with B1 (on the desk).
10) copy B1 to F. F is now the same as it was at step 1
11) remove and store B1 in the data safe. It is the backup copy of R.
I was once familiar with these shenanigans because the ICL 2903 also had one of these fixed and removable, 5MB per volume, drives. The fixed 5MB volume was usually the system disk, with the business programs and data on the removable 5MB volume.
The one I developed a system on also had a separate 60MB removable drive, so we only used the removable 5MB volume for backups, which was good, but this rig had its own problems because, while the 2903 was happy in an office environment, the 60MB drive wasn't - and the whole system was in a normal NYC office environment.
This was fine in summer because both people and the 2903 system needed cooling, but caused fights in winter because the office could only have heat or aircon on - never both - and so the ESD60 played up in winter.