Just for a bit of closure and to avoid hijacking this thread ...
The Windows 10 #1511 update issues on a 5K iMac is resolved. I WinCloned my external Windows 10 SSD over to the iMac internal SSD and performed the update there to completion. Now I need to decide if I should just call it a day and simply leave it internal ... or tempt fate and again WinClone it back to the external SSD (to avoid using fast SSD space, have 2x the space for Windows, and to keep it totally separate from the OS X disk).
Now ... per this thread ... my Windows 10 update seems to be running fine in my 5.1 Mac Pro as installed on a "external" SSD on a Velocity DUO in the PCI slot.![]()
I wish my Sonnet Tempo Pro supported booting Windows but it doesn't, regardless of Sonnet's mixed information on their site. But it loads stupidly fast on SATA2 anyway.