Boot up issues

[Gazcatraz]

Hi guys, I’m quite new to the pc build team. And I’m learning as I go, but today stumped me big time. 
I have an 500gb nvme drive that my pc boots from and has windows on, a 256gb ssd for further storage an 1tb HDD that is never used. 
anyway I decided to remove my HDD because I don’t use it and it’s noisy as hell. But now my pc won’t boot.... I’ve disconnected both my ssd and HDD to see the issue and my pc will only boot with the HDD installed. How has my pc managed to put important files required for boot on a unused drive and is it a case of wiping everything and reinstalling windows just to remove the drive?

thanks in advance for any help on the matter 

[maxdei]

i think think that could be one solution or make a copy of the files an put it in a partion with the same name on one of of the ssds

[Gazcatraz]

Thanks for your help, much appreciated