Since You mention that You have knocked over the Seagate external HDD have You run Seatools for Windows on it to make sure there are no problems?
The Portable drives get their power through the USB port. I have Used the Western Digital 2Tb Portable drive/ I paid $119 at the time, On Sale. Strangely when I bought, The Red was cheaper than the other colors.
I ran across this one while looking on the Internet from B&H Photo.
WD 2TB My Passport USB 3.0 Portable Hard Drive (Silver)
B&H # WEWDBY8L0020 ■ Mfr # WDBY8L0020BSL-NESN
You Pay: $109.00
Instant Savings: -$15.00
Vendors I would Shop, B&H, NewEgg, TigerDirect, and Amazon. B&H is in Manhattan, NY just outside the Lincoln Tunnel if You live near there.
I'm thinking of buying this one myself. As Well as the Toshiba, Despite the reviews on the Amazon Site, The Newegg reviews are better. I like to spread My data across different brands of drives. I'm using my last 2Tb USB3 Portable drive. I'm planning on using them to hold My Music Videos all on one drive. Right now they are spread out on 4 or 5 drives piecemeal.
No the Network Attached Storage will not auto mirror the data on your internal. That would be a software solution for any type of drive. When I say Mirror, I mean that the units I use have two hard drives in them and one is an exact copy (Mirror) of the other. If either one goes bad the Software that comes with them can send me a email notice to replace the failed drive. Once the failed drive is replaced it will rebuild the data from the good one to the new one. The only way to really lose data is if both drives were to fail. For example If the good one dies before I replace the failed drive.
The USB3 drives are faster, The other thing with a NAS is that I can sit at any computer on the network in the house and save to it or read from it. Thus it serves to hold the Music or video or pictures so that everybody can access them. Also the ones I have I can set them up so I can access their data over the internet if desired.
You may also have noticed that I plan to use a Toshiba and a Western Digital to hold the same data. That way if there is a bad batch from the maker it will not affect both of them.
You Might want to buy a USB3 Extension Cable if You need more than the 6" to foot long cable that comes with the drive. FWIW I use USB3 extension cables to make the rear USB3 ports on the USB3 card accessible from the front of the computer.