Your original HDD is also there, then it will boot from it.  
Hi everyone, I added SSD to M.2 slot and cloned my original HDD. However, I cannot set SSD as a default boot device in BIOS, have to use F12 (on-off) option. I prefer not to do this every time I restart the PC. Is there anything I can do? Thanks.
Dell 3280 AIO - would only boot from SSD in one-off mode