I tried to put a old hard drive from an old computer to a new computer but it is saying there is no operating system. The hard drive does have an operating system (windows 7) I don't know why it is not working please help
Your old Hard Drive has an OS w/ drivers from the computer it came out of. The computer you are putting in in has, I expect, has different . Doing what you are doing, once recognized, will have the OS trying to load hardware for drivers that aren't there and it won't find drivers for hardware that is there. This = FAIL.
If you want a trouble free OS on your computer, wipe the hard drive and do fresh install.
Windows 10 exists only as an equivalent to a OEM license..... if you take retail Win 7 you could install it 20 times w/o issue as long as it was on only one computer at a time.... **upgrade** to Win 10 and you lose that right, Win 10 is forever tied to the machine it's installed on no matter what you upgraded from .