Thumbnails on USB drive attached to NAS not previewing

From a fresh start up of the computer, I can get my thumbnails to generate any where on my computer, the NAS, or the USB attached to the NAS. However, after a short period (not specific, can be 5 mins or 30 mins), the USB drive attached to the NAS will no longer generate thumbnails, however, everywhere else still does (ie local files or even NAS files).

I have multiple machines, and it happens on this machine only (brand new only 2 weeks old, fresh windows 7 installed only 2 weeks ago).

I am using Windows 7 64bit Ultimate, but so are the other 2 windows 7 machines, and they do not forget to show thumbnail previews after a little while.

- "Always show icons, never thumbnails" from folder options is UNTICKED.
- tried all options from here Thumbnail Previews - Enable or Disable - Windows 7 Forums (with everything set to enable thumbnails)

What seems to happen is that it suddenly forgets that thumbnails are enabled, but only for that specific drive. Using the link above, it should always be the first example "EXAMPLE: Thumbnails Enabled in Windows Explorer" but when it decides to stop working it becomes "EXAMPLE: Thumbnails Disabled in Windows Explorer", however, it only effects that one specific USB device attached to my NAS.

It has to be a Windows 7 issue on this machine only, as all my other Windows 7 and XP machines always show thumbnails. As far as I know, all the Windows 7 machines have exactly the same software loaded (I re-formatted them all about 2 weeks ago when the new machine was built).

If I restart the computer all thumbnails are working again until it no longer likes me.

Any help appreciated.

It appears that all my thumbnails disappear after a time, but strangely at different times. First the USB attached to the NAS (usually within 10 mins of working with the drive), then the NAS (can take up to 30 mins, but as little as 15), and at about the same time, every other directory on my internal drives also stop displaying thumbnails, until I restart the computer.