Just sharing this for future reference, hopefully to save hair pulling for somebody along the line!
I built an OMV box using a large case and a known good motherboard, CPU and memory. I was combining 10 HDDs from 2 old boxes but all drives were known good.
I got a used Dell H310 SAS card from a reputable eBayer to host eight of the HDD's, connected the other 2 to the motherboard SATA and put the OMV operating system on a small SSD.
I soon was getting lots of problems which I couldn't make sense of - sometimes the system booted fine, often it got to the GRUB menu but then had lots of mdadm no arrays found errors and dropped out to BusyBox. I tried various fixes relating to GRUB in case that was corrupted, but no change.
Sometimes the system didn't find the OS drive at all. I tried changing power and data cables, but no change.
I thought there might be an intermittent fault with the SAS card, causing some weird conflict with motherboard BIOS, and this led me on a fruitless internet goose chase.
Finally it occurred to me that the SSD might be faulty. So I bought Crucial's smallest SSD, cloned the OS to that with Clonezilla and hey presto, everything working as it should.
What I don't quite understand is why the system, once booted, would run for many hours without fault - suggests that SSD only had an intermittent problem at power-up I guess.