Maximum memory on Ultra series

Because of the tendency of memory manufacturers to modify their memory configuration by changing the chips and arrangement, without changing the part number on the modules, Netgear no longer supports memory upgrades. You are basically on your own, but if you try it, be sure to run the memory test on the boot menu several times. (While the article mentions NVX/Pro, the Ultra line basically replaced the NVX and expanded it, and the Pro line is now expanded as well.

Unless you are running some application on the NAS, or have a heavy load of add-ons that do processing, you will find little if any improvement in the file transfer speeds. If you are experiencing file transfer speeds that are not up to your expectations, it may be a network item, a switch or even a cable that is not up to the gigabit standard. The Ultra should come close to fully saturating a gigabit connection in it's stock form.

If you feel that you still want to do it, I would recommend that you power off the Ultra, remove both side panels and top panel and the memory socket (laptop style) is under the top panel. Remove the module and note the make and model of the module and the full specs and CL or timing level. Then see if you can match the module, with the next higher ram level.