Maintenance and Service Guide  9-1
Addendum 108431-001 (11-88) to 
Manual No. 108033-003
9.1 THEORY OF OPERATION
The 300-/600-Megabyte Fixed Disk Drive  
Expansion Unit can be used with a COMPAQ 
DESKPRO 386-based Personal Computer with 
system ROM revision G or later. The expansion 
unit can be used as a normal external fixed disk 
drive for maximum storage or in a mirroring  
or duplexing mode. The three options are  
described below:
□ Maximum storage. This gives you the maxi-
mum storage space available in the drives.
Example:  With dual 300-Megabyte fixed 
disk drives installed in the 
expansion unit, you would have 
a total storage capability of 600 
megabytes of information. Two 
expansion units configured this 
way would give you a maximum 
of 1.2 gigabytes of storage.
□ Mirroring. In mirroring, two identical fixed 
disk drives are connected to a single con-
troller. Special software causes both drives 
to read and write the same information.
Example:  In this case, the same two 300 
megabyte drives would only 
have a total storage capability 
of 300 megabytes, since the two 
drives would contain exactly the 
same information. However, this 
in turn would keep the system 
operational and the stored data 
intact if the hardware for one 
fixed disk drive failed.
Chapter 9 
MASS STORAGE 
CONFIGURATION