Another point to note; QNAP hides the BIOS information log with a boot logo screen by default on the HDMI output. But unlike standard ASUS, Gigabyte, etc. motherboards, they've just used a black screen to hide the details. So unless your monitor is reporting "No Signal Detected" don't assume a HDMI signal isn't being output. Also, the HDMI connection is a negotiated link (i.e: bi-directional). So it needs to be connected, with the target monitor on and active the second the NAS is turned on. Later on down the line (after the NAS has hung) is too late. As soon as you start the NAS, start spamming the DEL / F2 keys like you would on a standard PC to get into the BIOS and turn off the BIOS "BootScreen Logo" or whatever it's called (I forget). Obviously, you'll need a USB keyboard attached to one of the USB2.0 ports, preferably a portable one with embedded mouse / trackball so you only need to connect one device. Don't attempt trying to use the USB3.0 ports in it's degraded state, as these might not be supported at the BIOS level properly.
These NASes are essentially just mini-pc type devices as far as functionality is concerned, and the "boot" drive (equivalent of the Windows "Boot Manager") is the internal USB-connected ADATA DOM chip. It exists to enable a "recovery" / "Install" enviroment for when fresh (i.e: bare, unformatted, unpartitioned, new) hard disks are inserted to install the QTS / Linux system onto. If this DOM chip gets corrupted, the system won't boot, and a recent version of QNAP's 4.4.1 firmware was causing this very issue on the TS453-PRO in a some cases - I can't remember the exact version, it was several months ago I fixed mine. I was able to recover my TS453-PRO using the method in the linked post above. I read the section of the guide listed for acquiring the TS453-PRO DOM image, but since the method for creating the USB stick for that model *doesn't* work (bad / incompatible display driver), I instead used the guide instructions for the TS453B-Mini, BUT with the DOM image I'd already downloaded for the TS453-PRO.
I've had no such issues on my TS453-Mini, but DID have to recover my PRO from the very same symptoms everyone else here is reporting. I did also have it setup to use HybridDesk Station (HDMI) prior to it going south though, which *may* or may not have a bearing on me getting HDMI output alot of others seem to be lacking (assuming the aforementioned boot-logo issue isn't the cause, as I suspect may be the case).
After the successful DOM re-flash, I then rebooted the device, and used QFinder to upgrade the firmware to the latest available, as with no drives installed, you have no web-interface or QTS system to access, and then rebooted it again. Once I verified it was starting with the latest firmware, I fully powered it down, re-inserted the disks, and powered it back up for a 3rd time. It booted into QTS without issue. However, I did have a FULL copy of all the data on the NAS, elsewhere, just in case.
Dan / Gib.
"Si vis pacem para bellum!"
NAS1: QNAP TS453-Pro (8GB, 4x4TB WD-Red), NAS2: QNAP TS453-Mini (8GB, 4x2TB WD-Red).