What I learnt from these forums is NO, you cannot boot from an external drive
I have my old laptop's had drive I put it in a case, I want to boot my new laptop off the old hard drive, so I can use the files/programs that are on it, both XP pro SP2. I changed the bios to boot from USB 1st, (this is what the case uses for the old hard drive) all I get is a flashing underscore with a black screen. When I boot the new computer up, and plug the hard drive in I can see all the files, and it works fine. Can you use this old drive to boot from?

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