The following was excerpted from an article posted to Info-VAX.  Note
that to use the DUA4xxx device with PCX, you should just use the command

    $ ALLOCATE DUA4xxx PCX_DISK

before running PCX.  Also note that you may not be able to just make
the second of the two drives available (with SET xxx HOST_ACCESS); I could
only make the second drive available if I made the first available as well.
-mdm

==============
>From: CLAYTON@XRT.UPENN.EDU ("Clayton, Paul D.")
>Subject: Thoughts On Topics Concerning HSC Usage, Problems And Trivia...
>Message-ID: <8912040431.AA25467@remote.dccs.upenn.edu>
>Date: 4 Dec 89 03:33:00 GMT

[...]
The use of FAKDSK will not work for CRONIC versions beyond 3.70. The alternate  
method is to do the following steps to gain access to the HSC load media from a 
VAX processor. The use of EXCHANGE is not needed here.

At the HSC console, with the switch in the 'ENABLE' position do these 
commands. The media must be write enabled.

^C
HSC>RUN SETSHO
SETSHO>SET SERVER DISK/LOAD
SETSHO>ENABLE REBOOT
SETSHO>EXIT

Question about whether a reboot can take place goes here...

After the HSC reboots, enter a 'SHOW DISK' command at the 'HSC>' prompt. The 
load media devices will show up with a device name of 'D4nnn'. One for each 
device. The next step makes them available to the VAX CI based processors.

HSC>SET DEVICE D4nnn HOST_ACCESS

This can be done for both devices. Write protect the boot media for the HSC at 
this point, and remove them from the drives, either TU58 or RX33. 

Next you want TU58 cartridges or RX33 floppies (already formatted using the 
RXFMT program) which are to have the new files copied to them. From any CI 
node the following commands will copy files as needed.

$SHO DEV DUA	(looking for a DUA4nnn device name which are load devices)
$MOUNT/FOREIGN DUA4nnn	(where nnn is from the above SHOW command)
$COPY/WRITE_CHECK file_name.extension  DUA4nnn:
.
.
.
$DISMOUNT DUA4nnn:

The device name may be prefixed with a HSC name such as HSC000$DUA4nnn:. Once 
the copies are made and depending on what you copied, you can then procede to 
configure the new tapes as needed. The HSC devices will remain defined within 
the HSC using this procedure, even after a HSC reboot. You WILL have to 
reenter the 'SET DEVICE xxx HOST_ACCESS' command when you need to gain access 
again. This does not require a system reboot this time.

[...]

Paul D. Clayton 
Address - CLAYTON%XRT@RELAY.UPENN.EDU

Disclaimer:  All thoughts and statements here are my own and NOT those of my 
employer, and are also not based on, or contain, restricted information.