
If their site thinks you're not on Windows, they'll offer you direct download links to the ISOs valid for 24h.

Click on the win10 image, then click Start in the top left of the Virtual Box. The reason is that if they see you use Windows they'll offer you their downloader EXE, which can only download to drive C: and if it's full (which is usual when using a small SSD drive as the main one) you can't download even if you have a secondary drive with several free TBs, since you can't choose another drive. The next screen will give you two options to download Windows 10. I used this one for Chrome: (User-Agent Switcher for Chrome) You just have to either use a non-Windows PC (Linux, Mac, Android, whatever.) or use a user-agent spoofer tool so Microsoft's website thinks you're not in Windows. There's another option to download the ISOs from Microsoft.
