“Windows 10 can create secret partitions on your HDD”
No it can’t.
Where do you people dig up this nonsense anyway?
It is known to sometimes create a recovery partition when W10 is installed but it can also create one or more partitions (possibly in error, but still) on your HDD while it is still running W7.
Easiest way to check is to let W10 download to your HDD and then see if you have any new partitions afterwards.
Then tell me if you still think it’s nonsense.
Actually it does in the sense it can create a backup partition that is not normally visible.