re: 6 month old Surface Pro 5 disk (C drive) is almost full... help!!

Try WinDirStat to see the largest files and folders on your system.

A quick thing to try is C:\Windows\Temp to make sure something isn't filling it up. Our old A/V was bad about filling the folder up when the updater would corrupt.

If this was an upgrade you can delete C:\Windows.old (but be warned that there will be some drivers that point to it, so they will not delete until you reinstall them and you won't know which drivers until you try deleting the folder).