I have a duel boot set up on one of my computers using two drives, windows 10 and windows 7. Recently, I haven't been able to boot to windows 7. I eventually found I could unplug the windows 10 drive until windows 7 boots, but I think constantly plugging a hard drive into my computer while it's on isn't a good idea, even though the bios supports hot plugging drives. On another windows 10 computer I have, I am unable to boot to a flash drive on that computer, but I can on another computer without windows 10, I have biited from flash drives previously with that computer, with the same flash drive, but I can't unplug the hard drive in that computer.
I have turned off safe boot, changed boot order, manually selected the drive and even used a boot loader made for windows. Nothing has worked on either computer.