IBM Midrange System DS4000 Series Hardware Manual page 82

Starting with controller firmware Version 7.50, the DS5000 supports the 448 drive
configuration. It requires you to attach seven expansion units (EXP5000) to one redundant
port channel pair. As a result, you will have four EXPs connected to one drive port pair and
three EXPs to the other drive port pair of the same channel, as shown in Figure 3-38.
Figure 3-38 DS5000 with 28 expansion units
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IBM Midrange System Storage Hardware Guide