The target drive on Windows 7 and 8 has to be at least the size of the previous one, which includes even the partitions which aren't copied. It seems nothing changed on 10 regarding that.

No, like I said, it's exactly the same. If it decides it's a system drive, you can't unselect it, and it's very easy to make something a "system drive" when it's really entirely non-critical (a drive with some games on it, for example).

I at least found the one tiny non-critical service I had installed from my downloads drive - a PS3 controller driver. Ironically having removed that, the drive in question is now impossible to include in a backup. WTF.

It's all very frustrating when you just want to make an image of a single drive.