Dual boot windows 10 and windows 8?

   Hi. No, not even if the same laptop hardware is in the new laptop, wehich would be the only time You could actually boot either Win8 or Win10.

   The Win8 installation is for the old laptop hardware - meaning drivers installed won't work unless the new laptop is the same model & hardware.

   The other concern is that Win8 would detect the newer hardware & probably flag the regcode as illegal, requiring a call to MS.

   However You CAN install the Win8 system as a VM running under Win10 :

https://www.youtube.com/results?search_query=pc+to...

   OR You could run a Win10 update on the Win8 driver installed into the new laptop, & it should use that regcode for an upgrade, converting the software * all licenses for it, into a Win10 software installation.

            G'Luck!!!

Maybe if you dual wield both components and blow one your rear end and the other up your frontal end? Oh and then gargle the third end from the top? Those always seem to be the best ways to help blow hards. Award best answer. If not blow it up your ****.

Yes you can, as long as the BIOS allows it.

"Could I boot both versions at the same time with the HDD used as an external drive?"

No. You can only boot one operating system at a time. You can have them both connected at the same time and select one to boot from when you turn on your computer, but you can't have two operating systems running simultaneously, or use programs installed in Windows 8 while using Windows 10 (unless they are portable apps).

If you need to run applications from the Windows 8 drive while also using Windows 10, you'll need to set up a virtual machine with raw disk access to the external drive.

I've never tried running Windows 8 from a USB-attached hard drive, but I imagine it would be terribly slow. It's probably not worth messing with unless your laptop has an eSATA connector and you have the appropriate enclosure.