If you are talking windows OS, it won't let you boot it from a removable drive. You'd need to put the drive into the SATA port where current DOS drive is instead. It may boot then, but almost immediately you should get an activation needed warning too.
Hello I'm sorry because I don't know much about computers but here goes... My laptop died and I bought a new one though it is DOS laptop. I was wondering whether it's possible to get the hard drive from the old laptop and to connect it externally and boot the new laptop from there.