Just put a the hard drive you want to use for 8.1 into an enclosure like this one.
Using Mint and qparted, make a partition big enough to hold all of your data, but not hog up your entire hard drive. Format this partition as NTFS. Preferably, put it at the end of the drive.
When you install 8.1, make sure that you do NOT use up the whole drive. Install Windows onto the unpartitioned part of the drive only.
Using Linux Mint, shrink your partition on your Mint system and make a new FAT or NTFS partition big enough to hold your data using gparted. Copy and paste your data using dolphin to the new partition.
Install 8.1 on your new blank hard drive or new computer.
Put the Mint drive into the enclosure and move your data. Windows will NOT be able to read the linux partition, but will read the FAT or NTFS partition with your data on it just fine.