I have been testing Windows 8 (W8) Enterprise Windows-to-Go
(W2Go) on a WD passport enterprise (500GB) external Hdd.
Using W8 I installed W8.1 Enterprise & got it activated, then installed the Hyper-V
In Hyper-V I installed W8 PRO & activated itv w/ my TechNet key then jumped through many hoops to get the internet working. Finally was successful & installed around 30/62 WU updates then checked the 'Store' & there was a link to upgrade to W8.1, ran that & it installed easily but took 1 hour, 41 minutes to complete the 3+GB download & upgrade, it's still activated. I do need to dump the windows.old folder but that's easy by running W8.1 'disk cleanup'.
So now we know that Windows 8 x64 PRO can be upgraded to 8.1 in Hyper-V on a Windows-to-Go (certified) drive
*Warning W2Go won't install W8.1 on a non-certified drive (I tried & tried & tried & tried several ways, incl. using imagex.exe but failed w/ non-certified USB3.0 drives). The imagex.exe W2Go steps work great installing W8 though.