Proseries data, like QuickBooks data cannot sit in Dropbox or similar since it is an open database file. 

I have use RAID (Redundant Array of Independent Disks) on my computer for years as another method of backup (if one hard drive goes bad the other one is available and up to date) and I've let the computer experts set it up.  Never had to use it so I "assume" it works. 

My suggestion is to open the file from the main hard drive and only use backup in case of need.  But, yes you can put a client file on a USB drive and operate from there with no speed differential - obviously USB 3.0 is faster than 2.0 and faster than 1.0.

It's not so much that NAS doesn't work; it's that you cannot use it as NAS, which sort of defeats the purpose for having NAS. NAS has no OS, with the concept that it is easily shared. But these Intuit databases are managed by their UI and must be managed by something that has an OS, because the OS runs the program (QB, ProSeries, whatever). This is a Network function, not a NAS function.

NAS is like a group of people that don't need supervision and still willingly and freely share.

Networking is like a group of people you are not allowed to wander in and interact with directly; you have to go through their Supervisor first.

In your experience USB 3 has the same (~4 second?) latency as a SATA connection when opening & saving files?