Acer Altos R920 Series - Manual (Page 194)

Appendix B: Memory configuration 176
5 Set the Select Memory RAS Configuration to Mirroring.
6 Return to the Memory Configuration menu option.
• Make sure Current Configuration indicates Memory
Mirroring Mode. If the option indicates Current Memory
Configuration, you may need to install more memory to
meet the requirements of the selected configuration.
• Make sure that Memory Mirroring Possible indicates Yes.
7 Press F10 to save changes and exit.
8 Press Y to confirm. The server reboots to activate the changes.