A little tip for those planning to upgrade to Windows 10 v1809.I have an old Toshiba Satellite with limited storage space which I upgraded to Windows 10.
I now received a warning that I don't have sufficient storage space on the HD for the upgrade to 1809.
USB drive to the rescue.
If you cannot free up enough space on the primary drive to upgrade to Windows 10, you can attach a USB key (16GB or greater) or an external USB hard drive to the computer to finish the upgrade. Once you have attached a USB storage device, when the "Windows needs more space" screen appears, select the drive letter for the USB key or USB hard drive to continue with the installation.