nw.exe --user-data-dir .\data
C:\bag\PortableApps\tiddlydesktop\nw.exe --user-data-dir .\data
C:\bag\PortableApps\tiddlydesktop\nw.exe --user-data-dir C:\bag\PortableApps\tiddlydesktop\data C:\bag\PortableApps\tiddlydesktop\nw.exe --user-data-dir=C:\bag\PortableApps\tiddlydesktop\data If you're doing the short path ".\data" then you need to be in the directory above data when launching. But, you probably knew that ...
C:\bag\PortableApps\tiddlydesktop\nw.exe --user-data-dir=C:\bag\PortableApps\tiddlydesktop\data
Then becomes (when launched from a bat file inside tiddlydesktop)
nw.exe --user-data-dir=.\data cd /D %~dp0
nw.exe --user-data-dir=.\data