Rolling back seems to be a good suggestion, if the menuing isn’t working for you.
But the drive should be being re-scanned if you re-attach it. That’d be the only way for new content to be added to the library.
If the drive stays attached during that power-on session, then the WDTV can keep using the existing Library because nothing will have changed, but if you disconnect the drive and re-attach it, or power the WDTV off and then back on, the WDTV would have no clue what’s been added or deleted. It can’t just assume that the existing library is still valid.
Even if it were to check the dates on the files to see if any are newer than the last library compilation, that would still involve scanning the drive to look for the files’ dates, wouldn’t it?
For that matter, if you buy 2 My Book Essentials drives, because they’re bulk formatted at the factory and a whole batch will have the same identifier, there’s a good chance that the second one doesnt initially show up under Windows, because Windows sees it as the same drive attached twice – you have to either re-format the drive, or force Windows to mount it. So, if you haven’t had a new drive identifier issued under Windows, the WDTV would have no way of knowing whether the “same drive” is really the same physical device, or just the same identifier on a different drive. But I don’t think the WDTV uses the identifier at all… I think it just sees “new device connected” and scans it for any changes.