In my opinion, like most PC users, I believe you would be much better served by purchasing a USB external enclosure + a HDD and/or SSD of your choice for your backup/storage strategy rather than a commercial "one-piece" unit.
The advantages of doing so are significant. First of all, virtually any USBEHD enclosure you purchase will accommodate both 2 1/2" & 3 1/2" drives. That's a real advantage. Then you can purchase the HDD (or SSD) YOU want, not the HDD that's installed in the commercial product. Rest assured the drive you purchase will invariably prove superior to the commercial product. And you can select whatever disk capacity YOU want. Another advantage is that the HDD you install in the USBEHD can always be easily removed and installed in a PC should that need arise.
Still another significant advantage is the fact that by using a separate USBEHD enclosure + HDD you will have UNLIMITED capability to utilize any number of different HDDs/SSDs for whatever purpose(s) you deem necessary or desirable.
All in all there are really no downsides to purchasing a USBEHD + HDD (and/or SSD) in comparison to purchasing a commercial one-piece product. And you will have an enormous choice as to the USB enclosure that appeals to you. One type is the so-called "docking station", see for example
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=blacx&N=-1&isNodeId=1
There are plenty of other types of enclosures. Right now, as an example, we're using the Orico 6518US3 model. See...
http://www.amazon.com/ORICO-6518US3-Super-Docking-Station/dp/B00CBPB4PM
Newegg also carries this model (they recently had it on sale @$16.99). And there are many, many others to choose from.
And it's child's play to install and/or reinstall a HDD or SSD in & out of a USB external enclosure - whatever the type.
(I should mention that many users employ a SATA-to-USB adapter device for connecting a SATA HDD or SSD to a USB port. Here's a sampling of those type of devices...
http://www.newegg.com/Product/ProductList.aspx?
Submit=ENE&DEPA=0&Order=BESTMATCH&Description=sata+to+usb+adapters&N=-1&isNodeId=1
Although I'm aware of many users who are satisfied with this type of device frankly, we're not. By & large our experience with these type of devices has been quite negative - we've found them to be defective or erratic in operation. We just don't use them any more. But I have to say we find that many users are perfectly satisfied with these devices.)
(I might mention that we have three of the Orico devices I mentioned above and all of them have performed flawlessly. Their dimensions are 7" long, 4 3/4" wide, and about 1 1/2" tall (at its highest point).
We have occasion to travel with them to other locations from time-to-time and with a HDD or SSD installed in the device we've never encountered a problem. Naturally one can carry the drive separately if desired since it's no problem simply sliding the disk in the unit for connections to the data & power connectors. The USB data cable is about 3' long so that should be adequate in most situations.
One thing else we like about this unit is that the power switch (button) is situated on top of the unit. So it's easy for the user to switch the unit either on or off. Many of the USB enclosures on the market mount the power switch on the back (rear) of the unit which I find quite inconvenient.
The power button is flush-mounted; I would have preferred a raised button. Also the power LED is not particularly bright; I would have preferred a brighter light. But those are relatively minor quibbles.
Many users prefer the so-called "docking stations". They're fine if one likes that style - (we have three of them hanging around that are rarely used). My chief problem with them is their bulk although I have noticed the newer models are getting more compact in size. It's really a matter of user preference. By & large they all do the job.