You mention trying to operate several devices. It unclear which combination works or doesn't work.
When I have a computer that does not start up, I try to track down the problem by removing all unneeded devices. Things that I can remove are the hard drive, ram, external devices. The aim is to discover which central parts of the computer still operate. I have emergency boot CD's which contain utilities.
I change one thing at a time. If something works then I add something else, or switch things around, until I find a component that doesn't work. It might require several shutdowns and startups.
If the hard drive cannot be read, then I start up from a different disk, or different OS, etc. Often I allow only one disk to be the startup disk. If I allow it to choose between two disks, I can't always know why it rejects one and chooses the other, or whether the other was the default, or which.