Hi all,
Thanks for all your efforts.
The first draft of the toolbox was built in Arc 9.3 (university-owned computer). Students have little control on when university-owned computers are upgraded; one day I walked into the lab to continue where I left off to find Arc 10.1 on the computers and my toolbox 'corrupted'. I cant remember what exactly I did to recover the toolbox but at the time my wife's work computer still had Arc 9.3, and I managed to somehow retrieve/convert/save the backed-up toolbox (since, as seems to be the case now, the working tbx was 'corrupted').
It seems that the issue is repeating itself here, the differences being different ArcGIS versions, and this time I do not have access to the previous version of Arc (I bought a home version of Arc 10.3 to enable me to publish my dissertation). So, there was no 'upgrade' per se, but rather a new installation. The last time I touched this toolbox it still worked, using the same name and the same location on my external as now).
My other toolboxes seem to not have these upgrade issues - just this one, now for a second time. Once it touches Arc10.3 it's empty. All toolboxes were made the same, in Catalog, before I started building them, so I don't anticipate that my methodology is the issue. If I recall correctly, all the tools in the models were also standard. Maybe there's a tool in there somewhere that's corrupt, messing up the entire toolbox with upgrades? (strange). It will be nice to know so I (other users) dont make the same mistake!
I'm currently out of town, without my backups. But I anticipate to be back home end of the week, in which case I'll attach the backup here somewhere.
Thanks again for all your help,
Johnny