High Sierra boot fails after 3 secs on previously running system - HELP!

[cloin]

Hi all, I had until this morning a perfectly stable running system - High Sierra with 10.13.6 running on an SSD. After installing a new NVMe SSD using a PCIe x4 extension board I was able to initialize the new disk, format it with APFS, copy 500GB of files to it and all seemed well. I decided to see if the new disk would affect the clover boot selection as I have had issues with extraneous disks showing up there. Note: I do not want to boot from the NVMe disk.
Upon rebooting the machine clover comes up and I select my usual ssd to boot from. I had changed nothing in BIOS or any EFI or other config files. However the clover now takes ca. 20 seconds "testing hardware" then another 30 sec. "scan entries" before presenting the disks. Selecting the usual disk results in a black screen with apple logo, and after 3 seconds the machine reboots. Using clover, spacebar -v and so on I get a bunch of output ending in Beginning / End RandomSeed and then ++++++++++++++++++++++++++++++, then the machine reboots. The same happens if I try to boot from USB stick with the High Sierra installer on it.
I have removed the new SSD, all of them but the primary boot disk, re-seated all the RAM modules etc. but to no avail.

Any suggestions on how I can progress with debugging this? I removed the RAM modules two at a time to test them, in case one has gone awol, but it seems they are all ok (BIOS finds them and shows them as healthy).

 

[cloin]

I finally managed to get my USB stick with an installable version of High Sierra to boot, using safe mode, and figured that the ssd I was trying to boot from had issues with corrupted inodes. I ran the disk utilities from the bootable stick a couple of times and then I was able to boot into the ssd in safe mode, check out that everything was working and then reboot normally. Phew! a couple hours of sweat. I managed to break the power connector off my 1TB 2.5" internal drive in the process (grrr) - I hope to glue the plastic bit back on that broke off, and then I will CC my boot disk to a second SSD just as a further backup. The NVMe SSD works fine. Really fast, too.