JBOD is as bad as RAID0 for data storage purposes - if you lose either drive you can kiss ALL the data goodbye.
I also get rather upset when I see comments like "RAID is NOT what REALLY works" - do your homework, find out what the product or feature is supposed to do and how it should be used, and then make an intelligent decision.
A NAS is designed to be used for storage to be accessed via a network, and in most cases by multiple users - if Acronis has a problem with writing images to a NAS, then take it up with Acronis - although - I can't help but wonder if the reported "problems" are actually Acronis related (since you say it sees and talks to your USB external drive perfectly) or to the network across which it's being used.
Wireless networks do not handle bulk data transfers gracefully, and you obviously have images large enough that a 320GB external drive is inadequate. The last backup I did with Acronis was a 65GB image - not something I want to transfer via wireless at all.
There are different RAID levels, intended to fill different needs, and RAID, in general, works - I've been using RAID in some form or fashion for close to two decades, and will not consider implementing any server or storage solution which doesn't have redundant disks.
For what it's worth - my data is stored on a server with a RAID5 array, and backed up to a DNS-323 with a RAID1 array