As with earlier posts in this thread,I can't over emphasize the value of an up to date backup and the use of fault tolerant storage.At minimum,Raid1(mirrored) preferably Raid5 .If using a NAS with a raid controller,buy drives rated for that kind of use ,not a standard desktop drive. Enterprise grade drives are your best bet ,they are made for being "hammered" day in and day out.It will cost more,probably triple what you would pay for the equivalent standard drive. Assume that at some stage you will have a NAS failure .Whilst a single drive failure can be easily recovered if you have a compatible drive to add to the array ( which in my case means keeping a stack on the shelf waiting for the inevitable ) ,If you lose a controller then the whole array is potentially lost. A desktop backup drive that is used just for backup and restoration is your second tier of defence, and a duplicate of that stored off site doesn't hurt.
I know this sounds a bit over the top,but if you have a big online music collection,it's worth a few dollars over the time and cost to recover from the loss.