it should work but also try to remove the GPT hard drive and boot with only the External so no duplicate Boot files which can make an issue you should try it (i also had 2 drives but one was GPT and one MBR and both was internal i disabled the GPT then installed windows then clean install on GPT and it worked fine )
Cut through all the BS, just install windows enterprise in GPT, and if your GPU or IGPU has GOP, you'll be in pure UEFI or not you'll be EFI and use KMS. People have said that the MBR to GPT works, but I've tried and keep getting ACT error, so instead of playing games, go Enterprise. Lose nothing and gain a few.
easily way and save time is copy some data from the laptop harddisk... get a 2tb external if your porn are full... just run an upgrade from it... many super help with activation here... get ori license also end up with pro or what ever... we on enterprise...