This is more of an unexpected "what happened" than a question so if it happens to you, you won't panic.
New DS920+ with 4, 8TB hdd and 4GB of internal RAM. 1. First memory test with 4GB ran as expected. Started it from the "Assistant" and the NAS went into reboot process and then started the memtest. After it was done, the NAS rebooted and everything was normal. Used PuTTy to SSH into the NAS and used the command sudo cat /var/log/messages | grep 'Memtest' to verify the memtest results. Test passed as expected. Ran two more times - all good.
2. Installed 4GB stick of Crucial (CT4G4SF8266) and started the memory testing process again. This time when it started the initial rebooting - the NAS shutdown. No LED's, no fans turning - nothing. After waiting several minutes, I long pressed the Power button and the unit started up and went into the memory test. Checked the results, everything passed. Performed the test again two more times and in both cases the reboot worked as expected, went into test, completed and rebooted.
3. Next I have a 16GB stick of Crucial (CT16G48FD8266) that I installed and started testing. Just like the first test on the 4GB stick, the first test on the 16GB stick failed to reboot and went into a power off state. No LED's, no fan's. Waited several minutes and then pressed the power on button but this time nothing happened. The power on LED would not turn blue no matter how long I held it pressed. Had to remove the power cord, waited 30 seconds and plugged it back in. Long press on the power button and the NAS came back up and started the memory test all on it's own. Tested the 16GB two more times, log shows all the testing for all the memory tests passed.
I have dumped all the logs after running the tests and will look for any event's / error messages to see if this is normal operation or a bug...
I have done some research and found this behavior being alluded to in a Synology forum for a different model but no replies or any follow-up posts.
I have exercised the NAS in each memory configuration after doing the memory testing and there was no abnormal operation, error events or anything else to indicate there was a problem with memory. The NAS enumerates and allocates the added memory and shows everything is as it should be. I have been playing with a number of app's to see if anything fails and so far, everything is good.
So if you experience this sort of operation when running the *first memory test* on your new upgrade, I wouldn't get to excited as long as the memory passes it testing and you're not getting any error messages or other weird, unexpected behavior.
DSM 7 is far from perfect. I'm running it on my other two NAS units (DS115j which I use for target practice and a DS718+ which I use for nightly backups for 30 computers) and I've seen some abnormal behavior in Hyper Backup (corruption) on the DS718+, screen display weirdness on the DS115j and a few one-off events on both of those that I could not duplicate. The DS920+ - with the exception of the first time memory test weirdness, has been running great.
No answers for that question.