HDD not recognised on boot

I am setting up an NAS using the Omnia 2GB RAM board and the NAS box.
I put two 3.5" HDDs (WD Red) into the NAS box. On booting the router, sometimes both drives are recognized, but most of the time only one is. The drives then do not show up on lsblk or in /dev/sdb. Both drives are brand new and fully functional. I also checked the SATA and the power cables.
What could be the reason for the drives not being recognised? Paul Totterman mentions a discussion in IRC (which I haven’t read) which suggests an insufficient power supply. Could that be? What alternatives are there?

Unplugging the second drive and plugging it in again when the router is fully booted will always get it recognised. It’s not ideal, but at least I can work with it.

The system requires a DC supply, throw away the " power supply " ( which is current limited ) and attach a battery bank such as a standby for a isolated modem. This will be a proper stable current manager.
Goes without saying the Battery bank needs to be continously charged by a proper charger. This also isolates from flash over voltages such as lightning storms, one of which is happening to me right now here in tropical Australia.

One of my TOs will be plugged directly into my Boat’s No. 2 DC (12v) Bank.