On some systems with an SSD set up as the boot device, if you later plug in a HDD the BIOS MAY find that new device and assume that it should really be the boot device and set itself that way. Of course, that could only work if the newly-added HDD actually has a Windows OS installed on it.
To fix that, you need to set the BIOS Boot Priority yourself, and to do that you need to get into BIOS Setup. Check how that is done with your mobo, but the most common way is as SOON as you turn the system on, you hold down the "Del" key. Instructions often say "tap" or "push" that key, but I find the system may not be watching the keyboard at just the right moment, so I hold the key down until the opening screen of BIOS Setup appears. THEN go to the screeen where you can set the priority of boot devices. Make sure that your SSD is the first. Then make sure that any device that you do NOT want to boot from (e.g., that Seagate drive) is NOT in the priority list at all. That way it will never be tried. When you have your settings right, remember to SAVE and EXIT to make those settings permanent. The system will reboot and should do so from the SSD with no other glitches.