External Drive Backup

Neither one.

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. And it's simplicity itself to install and/or reinstall a HDD or SSD in & out of a USB external enclosure - whatever the type.

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.

I assume you're familiar with the major online vendors, e.g., Amazon, Newegg, and a slew of others. You would be wise to peruse their offerings and select the type of USB enclosure that appeals to you. There's a wide variety to choose from, most of which will accommodates both 2.5" & 3.5" drives.

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.

Take my advice and go the route I suggest; you won't regret it.

1. Answering your second question first...there is NO warranty issue when you use a HDD or SSD as the drive in a USB external enclosure. Whatever warranty applies to the HDD or SSD still exists. Using the drive in a USB enclosure does not void the drive's warranty.

In this connection, you mention "building an external drive". There is really NO "building" involved. Have I not made that clear in my initial response to your query? All the user does is simply insert the drive in the USB enclosure. And if & when the time comes for the user to remove the drive from the external enclosure he/she simply slips it out of the enclosure's case. There's no complicated process involved. We're talking about seconds of a user's time here...do you understand that?

2. I really can't advise you as to which type of USB external enclosure to purchase. It's really just a matter of user preference. Virtually all of them are equipped with a A/C power adapter (with the exception of some small, portable-type enclosures specifically designed for 2.5" drives).

You might, for instance, be interested in a "docking station" type. I notice today's Newegg ad has one for sale selling for $13.99 (after a $11 rebate). See https://www.newegg.com/Product/Product.aspx?Item=N82E16817182311&ignorebbr=1

It's just one of many. I indicated some links in my previous response, did I not?

1. Answering your second question first...there is NO warranty issue when you use a HDD or SSD as the drive in a USB external enclosure. Whatever warranty applies to the HDD or SSD still exists. Using the drive in a USB enclosure does not void the drive's warranty.

In this connection, you mention "building an external drive". There is really NO "building" involved. Have I not made that clear in my initial response to your query? All the user does is simply insert the drive in the USB enclosure. And if & when the time comes for the user to remove the drive from the external enclosure he/she simply slips it out of the enclosure's case. There's no complicated process involved. We're talking about seconds of a user's time here...do you understand that?

2. I really can't advise you as to which type of USB external enclosure to purchase. It's really just a matter of user preference. Virtually all of them are equipped with a A/C power adapter (with the exception of some small, portable-type enclosures specifically designed for 2.5" drives).

You might, for instance, be interested in a "docking station" type. I notice today's Newegg ad has one for sale selling for $13.99 (after a $11 rebate). See https://www.newegg.com/Product/Product.aspx?Item=N82E16817182311&ignorebbr=1

It's just one of many. I indicated some links in my previous response, did I not?