Need recommendation of external HDD selection

Sure; here's mine...

In my opinion you, as well as most PC users, would be much better served by purchasing a USB external enclosure + a HDD/SSD for their backup/storage strategies 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 usually 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 significant advantage is that your HDD (or SSD) that you install in a USB external enclosure can always be easily removed and installed in a PC should that need arise. Keep in mind that many of the commercial "one-piece" USB external drives are equipped with a non-standard SATA data connector on the enclosed SATA drive and as a consequence cannot be connected to a standard SATA data connector of a HDD or SSD should that need arise.

Still another significant advantage is the fact that by using a separate USBEHD enclosure + HDD (or SSD) you will have UNLIMITED capability to utilize any number of different HDDs/SSDs for whatever purpose(s) you deem necessary or desirable. So you achieve an enormous degree of flexibility by purchasing the separate components referred to.

All in all there are really no downsides to purchasing a USBEHD + HDD (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
and there are literally scores of other models you can peruse.

There are plenty of other types of enclosures. Right now, as an example, we're using the ORICO 6518US3-V1-US model, a USB3.0 external USB 3.0 device that accommodates both 2.5" & 3.5" drives. See...
http://www.newegg.com/Product/Product.aspx?Item=9SIA1DS0FR8421&utm_medium=Email&utm_source=EXPRESS072316&cm_mmc=EMC-EXPRESS072316-_-EMC-072316-Index-_-HardDriveEnclosures-_-9SIA1DS0FR8421-S2A1D
Again, an enormous number of different models are available.

These USB external enclosures of one type or another are frequently on sale and generally obtainable at modest prices.

And it's simplicity itself 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

Then there's 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. And virtually all of them accommodate both 2.5" & 3.5" drives.
See for example http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=blacx&N=-1&isNodeId=1
and there are literally scores of other models you can peruse.