On startup: "No Operating system found"

[alexhoffman12]

Hello all,

I recently jerry(jury?)-rigged my build to use a selection of parts from a second computer that I got as a gift. The second computer is(was) a lenovo thinkcenter M800.
At the current stage, I essentially have simply moved the motherboard with all connected components into my old case.
Now, when I try booting with the harddisk it came with, all is well, and it has windows 10.
When I tried changing the SATA plug to my old harddrive, it enters bios and asks to continue (does this either way because of a "missing fan" error. Nothing to worry about,) but when i continue, it comes up with an error that says "No operating system found."

The old harddrive (isn't literally old) was working fine yesterday. It was the only harddrive i had, and thus had (I assume, unless I am really missing something) windows 10 installed on it. I dont see how windows cpuld have been installed on anything else, unless it was somehow installed on some hardwired storage on the motherboard. (If that is even possible.)

Any information or troubleshooting steps I could try would be great.
If anyone would need more specs about any components, I can relay those, but it seems like a generic issue.

Thanks in advance!

 

[Prostar Computer]

Was the "old" hard drive that you recently migrated over to this Frankencenter being used on a different system? Or was it working on this system until today?

 

[alexhoffman12]

This "old" harddrive was on a different motherboard. It is now on the "new" motherboard.

But note that it was the only one on the old motherboard, and the old system was running windows just fine. (I assume off of this drive.) Where else could it be from?