NAS is Network Attached Storage. Strictly speaking, when I think NAS, I think device on the network that never leaves the network - storage for the network, not "portable storage". You're talking about cheap NAS devices which can be portable, but really, that's NOT what they were designed for.
RAID is a Redundant Array of Inexpensive Disks. The point of RAID is to guard against a disk failure by having a mechanism in place to ensure the data can continue to be accessed if one or more drives fail (depending on the RAID level). RAID 0 is, strictly speaking, NOT RAID - it provides for faster disk access because the data is spread over multiple drives, but there is no redundancy which means since there are more disks, the risk of failure (compared to a single disk) is actually GREATER. But so is the performance. Because RAID requires 2 or more drives it is inherently less portable. There are enclosures that support/create RAID arrays, but again, GENERALLY, they are NOT intended to be moved - they are intended to provide storage to other systems on a given network.
If you want PORTABLE storage, you should get portable storage - an external USB2/3/Firewire/eSATA drive. This would be a SINGLE drive with up to 4 TB of storage today. SOME external drives MAY be dual drives in a RAID 0 (Lacie had some of these) and there's even a possibility that you could find a dual drive portable unit that mirrors (RAID 1) - I don't know of any.
Most things the everyday user would buy can be considered portable... it's just a matter of how easily portable they are. Your desktop computer and monitor are portable... Add a really big UPS and you've made a REALLY big laptop. But do you really want to carry two or more things around and set them up every time you want to use the computer or would you rather carry something designed to be carried out and about like a traditional laptop, ultrabook, or netbook?
As for accessing non-traditionally portable devices while out - VPN and remote access solutions. But these are usually FAR slower than an external drive over USB (with USB speeds seeing typical sustained throughput of 180 Mbit and your average home internet upload speed between 1 Mbit and 10 Mbit (depending on your service and where you live). The reason you use an external portable USB drive instead, at least in my opinion.