you mean dual boot
windows has a very nasty habit of taking ownership of the boot process meaning writing things in places it should not.
Short version and simple version is to install a second drive, disconnect the linux drive, install windows , reboot as many times as needed to finish the windows install, shutdown reconnect the linux drive , in the bios set the linux drive to boot first, in linux update the grub boot loader et voila you have mint and windows ready to go (this was the simple version)
People disagree with me. I just ignore them.
(Linus Torvalds, regarding the use of C++ for the Linux kernel.)
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