Hi @gprovost and thanks for your quick reply!
It could indeed be the spin-up current, according to the datasheet (pdf) I've found at the HGST website the disks I am trying to operate require up to 1.2A @ +5V and 2A @ +12V - theoretically combined 2*30W = 60W at startup - which seems, according to a comparison-sheet by a dude over at FreeNAS, like quite a lot.
Let me answer your questions:
The exact model is called HGST HDN724040AL. I decided to opt for them instead of the classic WD Reds as HGST drives were sort of considered "the tanks" among HDDs in general (lower failure rates I were told, that stats might be of interest for some people considering which drive (manufacturer and/or model) to buy, note though: these are the statistics for Q4 2017 as well as 2017 in total, at this page there are quite a lot more observation periods available).
Indeed I did try both SATA plugs on each cable with both cables with both molex-headers on the board with both drives I have got here - already tried to narrow down the issue as good as possible (16 tries in total)
When connecting the SATA power cable while the SBC is running, the drives also don't spin up at all (seems like most of the time they don't even start spinning up as they generally have to me a notable noise)
Also notable: if the system heartbeat is illuminated in the moment the power connection from the board to the drive is established it remains on permanently, same goes for the case when it is off in that moment.
When powering up without any drives attached it works fine (boot-log when no HDDs are connected is attached)
If there is a single drives' SATA-power connected to the helios4, the system either does not boot at all and the drive does not start spinning up, or it stops fairly soon after starting to spin up the drives, which then also start slowing down again - looks like they then lack power as well.
If the drive starts spinning up and the system starts booting, the serial console (and presumably also the entire system) freezes at about line 20 (varies partially by some lines, I guess sometimes it freezes earlier, sometimes a bit later) of the attached log and that's it then, the mains power has to be disconnected for it to start again.
If one drive is connected at powering up (note: serial console remains clear) pressing the CPU-reset button does not change anything: the power indicator remains on, the heartbeat and the OS remains off.
It is possible to power up the system without any drives attached, which is also how I tested for the 2nd question (always connected the HDD once the system was properly booted and responsive). It instantly freezes, no heartbeat any more, the LED1 remains as it was the moment the connection to the drive was established.
As of right now it seems it's likely a issue with the spin-up surge current required by the drive(s), I'm gonna look whether I can activate the PUIS mode for my HGST drives - any advice is still very welcome!
Attachment:
helios4-no-hdd-startup.log
A short update (nr. 1): there seem to be multiple ways to prevent the drive from spinning up rightaway (PUIS as well as "SATA pin 11"), but right now after all the testing the board does not even boot without any HDD attached anymore (neither a system heartbeat, nor any serial output) - lol.