I wouldn't do USB recovery at this point, because the system is eventually booting. So it doesn't seem likely that this is related to corruption of the flash.
You should download the full log zip file from the admin web ui. Then ask a Netgear mod (for example, @JohnCM_S ) or perhaps @Hopchen (former Netgear) if they are willing to analyze them for you. Put the log zip into cloud storage (google drive, dropbox, etc), and send the link in a Private Message (PM) to them. Don't post a link to the logs publicly here. The PM facility is reached using the envelope icon on the upper right of the forum page.
Mine does that a lot, and also sometimes hangs before completely shutting down when I re-boot or power down (making the liklihood of not coming back up higher). When it finally comes back up, it seems the OS partition (md0) is always re-syncing (which you can only see from SSH, not the GUI). I always assumed that was because I "pulled the plug" to try again during a critical write operation. But maybe it's related to the root cause? Perhapos it's making it's way down the list of OS partitions (it's a RAID1, but made up of a duplicate partiton on all of the drives) on each re-try until it finds a "good" one.
