There probably is a limitation Pete, like car USB players they can only index so many files before it becomes a case of indexing takes longer than the length of the film you want to watch, all I can suggest is to use a folder structure if you haven't already and limit the amount of files in each folder so to hope that the USB can read say 1000 folders with 100 files in each a lot easier than one folder with 100000 files in it, I know my car is limited to something like 1000 files and if I add more files they won't appear at all on mission control in the dashboard.

Hannibal Lackluster wrote

There probably is a limitation Pete, like car USB players they can only index so many files before it becomes a case of indexing takes longer than the length of the film you want to watch, all I can suggest is to use a folder structure if you haven't already and limit the amount of files in each folder so to hope that the USB can read say 1000 folders with 100 files in each a lot easier than one folder with 100000 files in it, I know my car is limited to something like 1000 files and if I add more files they won't appear at all on mission control in the dashboard.

That's how I have it organised for the same reason, my car has the same problem. Thing is, I'm fairly sure that the very first time I plugged the USB HD in, everything on it appeared on the TV.

UPDATE I've had an email reply from TP-Link which confirms that there is a file limit !! -

"Thank you very much for requesting information about our product.

There is a limit on VR400 on Media Server sharing files. It can support at most 20,000 files, including the folders.
Please try the steps below to help confirm what's the root cause.
1. Please make sure the files are within the limit.
2. Media Server can only help share media files, like photos, videos and audios. Please check your file as well.
3. For the missing folder, please share them separately and confirm if you can view it under such situation.
Or you can also try put all of the files in one folder and check if you can view it successfully."

I think I'll use Plex then, much better interface on the TV anyway and no limitation !

Thanks TP-Link for not including this info in your specs !