How does removing or inserting memory modules affect my system?
Before adding or removing memory, disconnect the power cord and wait for about 30 seconds. Insert memory correctly into sockets labeled DIMM1 and DIMM3 on ProDesk 400/480 models; these labels differ for ProDesk 490 (Page 26).
From the official HP Prodesk 400 G3 Desktop Mini PC manual, page 26
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