IBM 5280 User's Setup Procedures page 119

48. Make sure all diskette 1 and 20 drives have been checked.
If your system has a mix of diskette 1 and 20 drives, be sure you used
one of the system checkout procedures (starting at step 23) for each
diskette drive.
Read the progress check block and continue to the next step.
PROGRESS CHECK
1. You should have keyed a 2 and a Z at each keyboard on your system.
2. You should have keyed the characters ABC ... XYZ and 012 ... 789 at each
keyboard on your system.
3. You should have moved the verification diskette into and out of
every diskette drive, and moved the blank diskette 20 into and out
of every diskette 20 drive on your system.
User's Setup Procedures 113