Please have a look at the following topic:
It is the same for Windows 7.
Wil| Author of Vimalin. The virtual machine Backup app for VMware Desktop Products
| Vimalin : Automated backups for VMware Fusion and VMware Workstation Professional
| More info at https://www.vimalin.com
| Twitter @wilva
| VMware Wiki at http://www.vi-toolkit.com
I followed this fellows instructions, except I used a registered version of Fusion 6 and everything went fine.
Downloaded the Windows10_InsiderPreview_x64_EN-US_10130.iso and no problem clean installing on an external drive partition USB3 connected to a Yosemite Mac Book Pro (Mid 2014), no video or accessory problems....
I usually buy the even numbered Fusions, so I'll get 8 when it comes out optimized for Windows 10.
And I'm thankful VMWARE supplies a solid product that works for at least two generations of operating systems.