Next plug the drive into a Windows machine, the exfat partition will show up, format it NTFS, download the Win10 Technical Preview ISO (noting the activation code), run Wintousb select the ISO, and simply select the same NFTS partition as system and boot. Then option boot to that drive and partition it using OSX Drive Utilities with an exfat partition. One way to do it on an external drive would be to simply use Carbon Copy Clone to clone your OSX to a larger drive. Otherwise the Windows OS will need reactivation any time you use it on a different computer. Look at Wintousb vice Wintogo! Wintousb is really easy to use, particularly with Windows 10 since there is a universal OS activation key good until about next spring. It can be done but there are several ways to do what you want and it will be time consuming, error prone, and the steps will be different depending on what hardware you have, the programs you use to do partitioning, cloning, and OS ISO's. The windows partition will have to be NTFS.

Wintousb unable to select partition