Won't boot SSD

[Zombysz]

Hello there. I am working on a client build, and I am facing an issue.
In BIOS, it sees my boot USB. However, it doesn't boot from it. It says the 'Boot device not found' error. Then I plugged in the boot USB in my own pc as well as his SSD, installed Windows on his SSD, and it worked fine. So I plugged the SSD into his mobo (and psu ofc) and guess what: The BIOS sees the SSD, but the same error. AGAIN. Here are some other things I've tried:
- Cleared CMOS (2 times)
- Pulled out a RAM stick

I hope you can help, here are the specs:

CPU: Ryzen 5 1600
MOBO: (which might be the problem here) Asrock X370 Killer SLI
RAM: HyperX 2x4GB
PSU: Seasonic M12ii EVO 520W

Thanks in advance!

 

[gdmaclew]

Does the new SSD have an OS on it?
The "Boot device not found" error means it doesn't.

 

[SkyNetRising]

Boot settings in BIOS have to match the boot media.
If BIOS is set to boot into "UEFI only" and boot media is prepared for legacy boot, then you won't be able to boot
(same goes, if BIOS is set to boot into legacy only and boot media is prepared for UEFI boot).