PROMISE Technology Inc.
17 January 2019
I recieved a reply to my query last night, and apparently there was some OEM work with NUUO many years ago, but I have no details other than that. And apparently, there was a Nuuo nvrmini, but my googling found only it's successor, the Nuuo nvrmini 2. Obviously you are correct that it is Promise hardware.
That being said, since it is an OEM product, support is through the OEM Vendor, NUUO.
Also, if the firmware is custom then the OEM would have different passwords. And chances are that you will not be able to load promise firmware as it will verify the vendor code from the file. This may be why your attempts to update to Promise firmware via the ramdisk have failed.
As a side note, loading firmware from the ramdisk was broken in later releases of the NS4600. The problem, the firmware files were larger than the RAMDISK. Also you can only upgrade NS4600 firmware, trying to downrev the firmware may have unpreditable results. So any firmware upgrades to the latest promise relase have to be done from the GUI.