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From the manual, Guardian OS:

Determining Capacity
The factory default configuration reduces potential capacity to provide a high
degree of data protection and backup capability. By default, Snap Server disk drives
are all part of the default RAID 5 created during the manufacturing process. In a
RAID 5 configuration, the capacity of one drive is used for data protection, reducing
the available capacity of the server by one drive. The GuardianOS runs from a
protected partition, which consumes approximately 10 GB of space from each disk
depending on the total capacity of the disk drive. Snapshots are active by default,
with 20% of the default RAID assigned to snapshot space and 80% of the default
RAID assigned to the primary data volume.
For example, to calculate the capacity of a Snap Server 4500 with 480 GB total
capacity in its default state, consider both the hardware and software
configuration:
• The four 120 GB disk drives provide 110 GB of formatted capacity.
• The four disks when joined in a RAID 5 configuration net 330 GB of capacity for
the RAID.
• The snapshot space is 20% of the space available on the RAID, reducing the space
on the RAID for the data volume by 66 GB.
This results in a data volume capacity of 264 GB for this Snap Server 4500/480 GB.
On a Snap Server 18000, all eight drives are incorporated into the default RAID 5.
On a Snap Server 14000, eleven drives are incorporated into the default RAID 5 and
one drive is preconfigured as a hot spare for the default RAID. The capacity of the
hot spare is not included in the total capacity of the volume or snapshot space.

Why are you installing Maxtor drives which have the highest failure rate of any drive on the market? Do you not value your data. You should move to the WD RE drives.