Window 8.1 (fresh install) > Windows 10 error /dynamicupdate

N times I am trying the upgrade to W10, all stopped by the same error, after uploading the media and starting the upgrade, an error popup up saying that an unknown command line option /DynamicUpdate has been used, and back to the W8.1 desktop!

I am also running into the VERY SLOW performance because I have my VM on external SSD, so it tooks AGES to run any command, which is VERY annoying.

I have looked at different forums and nowhere is the root cause of this error explained, another annoying fact but from Microsoft.

May be I should forget about running W10/Parallel on my Mac and buy an inexpensive PC for the few utilities that I need and running only with Windows.

Or is out there a genius Guru who knows exactly what to do, nothing like "may be try this or this", but with the exact reason why this error pops up and how to fix it for good?

Thanks!

 

I finally got a valid VM upgraded from W8.1 to W10.
For the performance, it is clear that all external SSD are not equal : the "integral SSD 512Gb" (bought from Darty) was the one with very poor performances, getting very hot while trying to run the VM on it. It was unusable even for other Mac OS tasks at the same time.
I tried another one, an internal SSD (Kingston) that I recycled into an Inatek adaptor (from Amazon), a tool-less model. And then all went smoothly, performances where back.
But the error still pops up (I cloned the W8.1 on it). My idea is that the windows 10 updates needs some room space and that to create this, I had to use some space then delete the files. I downloaded the W10 image for DVD (3Gb) then copied it on the Mac partition, then deleted the image in the VM (so now I had 3Gb free), mount the iso on the DVD interface of the VM, and launched the setup.exe. All went very well (and very fast).
Hope this will help if someone else run into the same kind of troubles.