AMI BIOS boot options?

Good idea, that is another method. I tried to move the external USB to the first position (as usual for a normal BIOS) and then disabled the rest. I will try your idea with keeping the external USB in the third position and disable the internal flash at number one and two.

So maybe I miss read you at first tigerten. Of course I tried disabling the internal flash. You can read about the attempts above.

I tried it one more time. I was curious about leaving boot order position 1 and 2 disabled and the external USB enabled in position 3. That did not work at all. With that setting the system booted directly into BIOS. In other words, there has to be a bootable device in position 1. I also tried my previous attempts (the most logical) setting with the external USB in position 1 and internal flash disabled in position 2 and 3. That resulted in same as before. The BIOS setting was overridden and the internal flash was set in pos 1 and 2. I also tested with having the external USB in pos 1 and the internal flash in pos 2 and 3 enabled. That also resulted in being overridden on next boot.

So the problem is still the same.

Did you do this?

"If you disable the first, the second will become the first. Disable that again, the third will become the first."

As I said, the order is built in the BIOS. you can not re-order them. Custom order will not persist. you can only elevate the one you want to the first by disabling the ones before. 

Yes. As I wrote before: having the external USB in pos 1 (by first disable pos 1 and 2, thus elevate what was in pos 3 to pos 1) will not stick on next reboot. And having pos 1 and 2 disabled and external USB in pos 3 will result in BIOS showing directly after reboot (without pressing DEL och F2).

Sorry, don't know what else can be done. mine is 628X, and don't think that will be reason. 

tbh, I don't think I follow what you did. 

I solved it!

I blame TinyCore for this.*

Today I tested to boot up OpenMediaVault (with same USB stick as for TinyCore) and it presented itself with both legacy and UEFI. I tested both and RN312 could boot from both OMV's legacy and UEFI and the setting stuck in BIOS! Now the machine boots up from the OMV external USB every time.

OMV boot could be place in position/slot number 3 and having internal flash in 1 and 2 disabled. Or setting OMV in position 1. So the external boot priority position could be elevated/moved, the critical thing was that internal flash was not enabled with higher priority than the external boot.

No other setting needed to be changed in BIOS.

This also explains why most users don't report any problems booting from external USB. They where lucky with a boot device that was fully compatible ReadyNAS OS's BIOS.

* Note: TinyCore only had legacy boot.