IBM x3550 M4 Type 7914 Installation And Service Manual page 304

Figure 176. Latch adjustment
Figure 177. Backplane removal
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IBM System x3550 M4 Type 7914: Installation and Service Guide
b. Lift the hard disk drive backplane up to remove it from the server.
If you would like to remove the 2.5-inch drive backplane.
a. Lift the hard disk drive backplane up to remove it from the server.