Compaq ProLiant Storage System User Manual page 28

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If you have installed the Fast-Wide Duplexing Option in your ProLiant Storage
System, proceed to the next section, "Duplexing Cable Connections," after you
have completed the cabling described here. If you are not using duplexing, skip
to "Installing Hard Drives in the ProLiant Storage System" later in this chapter.
IMPORTANT: All SCSI hard drives on the same SCSI bus must be either
internal (within the server) or in an external storage system, but not both.
A configuration with both internal and external SCSI hard drives requires
more than one single-channel SCSI controller. A multi-channel controller,
such as the Compaq SMART SCSI Array Controller, supports both internal
and external SCSI hard drives on separate SCSI buses.
Compaq ProLiant Storage System User Guide
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