You actually don’t need to do all the things your doing.
You don’t need the yourfilename.jpg in the xml, it’s not really used.
You also can create the moviesheets directly to the HUB (or attached HD) from Thumbgen. No need to connect anything to your PC.
Just do everthing you had done in creating the thumbnails and moviesheets then edit the xmls, then once your done, unplug the HUB power cord, then plug it back in and your moviesheets should show up.
The .tgmd file is an internal Thumbgen file (Thumbgen Metadata) which is used if you change moviesheets. This is mainly so that you can use “Automatic” mode if you decide to change the moviesheet you want to use.