Try this
On my 1tb portable I have a Steamapps Folder and a SteamAppsStable,
I wrote a command like app to simply check each status and copy ones at 100% to my Stable Folder.
Leaving the originals to be updated and messed with by steam.
Downside I have duplicate content, upside it works perfectly.
Personally its a pain because we pay per gig in my country for bandwidth.
So updates are 1 slow, 2 painful, 3 annoying specially when you click dont update this thing steam and steam goes huh what sorry wasnt listening I was busy ignoring you and deciding to randomly update files.
Also UCL loader seems faily happy to run games that are bein updated so long as they have been 100% at some stage.