15 R3, boot order management

Got a weird issue. I am in the process of selling my 15 R3. When I had it originally it came with a hard drive so I bought a M2 drive and cloned my OS to the new drive. It always then booted from the M2 drive despite the hard drive also still having the original OS. I took the M2 drive out today to place in new system but forgot just how slow a hard drive is when I rebooted the 15 R3 so put the m2 back into the 15 R3. Now the 15 R3 will only boot from the M2 if I select it in boot manager. Once I restart it uses the hard drive. I even formatted the hard drive but then it attempts a repair so is still being sent to that drive and not the M2. It worked fine before so not sure what's up. There does not appear a way of changing boot drive order. If I took the hard drive out would that solve the issue?