My primary, active (vs. backup) local disk space is ...

Not true! The disk space on Surface just takes into account that there are no worthwhile apps to install.

well, no not really. In a RAID0, there is no Redundancy. It's more accurately a AID, or just an Array of Inexpensive Disks. For Redundancy in a striped array, you need parity information to ensure against failure of *any one disk*, and that's a RAID4, 5 or 6 (6 giving you double failover with not one, but TWO sets of parity information distributed throughout the array, as opposed to RAID4 which has one set of parity information written to a single disk in the array and RAID5 has that single set of parity information distributed throughout the array).
In a RAID1, there is 100% failover redundancy but this is not a backup solution. All it is, is an insurance against physical failure of ONE HALF of the array.

If you use a RAID, you need to be aware that whatever you do to the data on the array, is what you do to it. IF you delete it in such a way as to render it irretrievable, then that's it. Unless you have a backup.

**REDUNDANCY is NOT BACKUP**

Or perhaps people don't bother deleting stuff they no longer need until they start to run out of space.