The only thing that might affect things is that the Foobar.xcodeproj file is not in the Foobar folder but up one level in the FoobarWS one.
i.e. /Volumes/Data/Development/MyProjects/FooBarWS/Foobar/…. the project files
/Volumes/Data/Development/MyProjects/FooBarWS/Foobar.xcodeproj
The project was originally created that way (on 6 Jan 2012 - as an Xcode 3.2 compatible one per the Inspector of the Project document) and I have not moved anything around.
Where is the .git file? In FooBarWS or FooBarWS/Foobar? In the normal course of events (Xcode-5 version of “normal”) it should be in the same folder as the project file, i.e. FooBarWS.
As a result of a similar discussion in the forums, I *was* able to reproduce your inability to add the repository. I finally got it recognized by using the repository add (in Preferences/Accounts), then quitting Xcode, then having Xcode re-launch and open the project. When the project re-opened, it was under source control. (The easiest way to check is to select the project item in the navigator, then use the file inspector to see the big green dot in the SCM status at the bottom.
If your .git file was one level lower (which was a configuration that did work in Xcode 4, I think, though of course the project file itself would not be in the repository), it’s going to be harder to “move” the .git file up a level, since literally moving it will break all the paths it contains. My git skills aren’t great enough to know how to fix that, but I suspect the trick is to use a ‘git rename’ in some form to add the extra path component to all of the repository contents, then to move the .git file.
I tried moving it into the Foobar folder but got a build failure that it could not find the Foobar.plist file. It is there in the Supporting Files folder.
Re-adding it to the project, cleaning and deleteting the derived data still produced the same error.
Yes, you can’t move it like that, because everything *else* the project contains is relative to the folder containing the .xcproj. If you move it, everything else breaks.