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I found a 'workaround' for this - on the Mac if you go into System Preferences (this is on Yosemite), then iCloud - select iCloud Drive - then Options - it lists all applications 'allowed' to use iCloud.

If you de-select Textastic - it seems to fix the issue - i.e. If you load textastic, it creates a new document, called 'Unititled 1'.

If you leave it there for a while - and go to quit textastic - it prompts "Do you want to save this file?" etc.

If you don't save it - it doesn't get saved.

Even though iCloud Drive is de-selected for Textastic this doesn't seem to stop you from loading / saving documents to the iCloud drive from within textastic - it just seems to stop the documents (even temporary documents you don't want to save) from being saved 'in the background' by default to iCloud.

This is obviously what you want if like me you're end up using textastic as a temporary clipboard dumping ground :)

-Karl