Thank you dillyhammer !
I have been having the very same problems with my 4000NV+. I am tring to view my movies on my ipad using "ACEPlayer" (A good little ipad app that uses DLNA to stream and play all video formats direct from the NAS).
It takes sooo long to scan (though I do have 7500 music, 350 movies and 1000 tv shows) and it seems to stop and hang during its scan half way through my movies folder (in alphabetical order) and doesn't scan in any further movies or folders.
Using your very helpful post (a lot more helpful than Netgear tech support), I found that it was stopping on a MKV file (7 GB), though NOT the biggest file I have, 10 and 11 GB movies scanned in just fine.
My simple way of finding what file it got stuck on was by using the free "8player lite" iPhone app, which allow you to quickly view the DLNA library, find the last movie scanned in, and then finding the next movie from the network share on the PC (in alphabetical order).
Delete the offending MKV off the NAS. And then stopped and restarted DLNA.
Yes, it had to rescan the entire NAS again,
took about 2 hours, but found all the rest of the movies and continued to find ALL my TV Show folders and episodes!
After it was completed, in my readyNAS frontview, under DLNA, I now have the "Rescan meda files" button available (always used to be grayed out while DLNA was running).
The very sad thing is because this is technically a currupted MKV file problem, Netgear may not bother fixing it. But it really can't be a difficult fix to simply have their scanner just skip over files it finds that it cant scan, instead of hanging indefinatly.