On the original system that won't post, try this. Power off the system, open the case and physical remove all hard drives from it. Next, physically remove all RAM sticks from the memory slots.
Yes, all of them! Power the system back on; if you don't hear short and long audible beeps coming from the Motherboard internal or Case Speaker, then your Motherboard on that machine is dead and needs to be replaced.
Moving the hard drive from the 1st system to the 2nd system will not work as the Windows is tied to the
EXACT hardware, CPU, fsb speed, etc. on that Motherboard and won't work properly, even if W10 appears to boot on it. (did you check your device manager to see if all devices were working so as disk storage, network adapters, etc. without warnings--yellow exclamation marks, red "X-es" etc.?). Similar specs or the same brand CPU do not count!
Installing any hard drive on the 2nd computer with W10 and trying to move it back into your original PC will not work either for the same reason.
As kemical, suggests you really need to do a Clean Install of W10 on that 1st computer with a known good hard drive.
Also, that message you mention is trying to boot from a network ROM server image, which you probably don't have, and it could be as simple as your BIOS not finding the hard drive in the boot order preference. Access your BIOS and make certain that the 1st hard drive is being read in the BIOS and shows up with make/model and capacity on the BIOS storage screen. If not, you could have a failed hard drive or a failed Mobo. The removal of all HDDs and RAM sticks is pretty much a dead giveaway as if that doesn't work you have a faulty Mobo and will have to replace it to get that computer ever to work again with any HDD. Or simply junk it and replace it with a newer machine.
<<<BIGBEARJEDI>>>