External 3.0 usb Hard drive

What is your preferred sales outlet? There's not a lot of actual difference. The drives inside are all pretty much the same.
Anything from one of the major manufacturers would work.

A 500GB or 1TB would suffice.

My individual experience is that external drives have been less reliable - but this could have been for any number of reasons, and the volume of drives isn't a sufficient sample to draw reliable conclusions from. In some cases external drives are prone to failure because they are poorly handled - or moved while in use (this is bad for traditional hard drives - pretty much a non-issue for SSDs). This is - in part - why non-SSD laptop drives tend to fail sooner than desktop drives - they're actively used while being moved about.

This said, I do tend to avoid external drives now and am happy with other advantages this has brought. Instead, I purchase internal SATA drives and use an external docking station (StarTech manufatures lots of these, but other companies - whom I've not yet used - do too).

Internal drives are almost always cheaper by the GB/TB than their external counterparts, and because they're not boxed into a portable shell/case (when used in a docking station, at least), they should run cooler. Note that because the drive is exposed you should take extra care when handling the drive to ensure that the board is not compromised/damaged by wandering fingers or anything else. I tend to purchase properly sized silicone sleeves for my drives in order to offer a little more protection (also available from StarTech). These sleeves also allow you to safely store the drives on top of one another if you are expecting to use a number of them.

Hitachi had a very poor drive manufacturing track record for a number of years, but perhaps in response to this I've heard accounts of their newer drives being some of the most dependable. My preferred internal SATA manufacturer is Western Digital - and there is a range to choose from;

• 
  ■The WD Greens are some of the cheapest, but they're not as durable as some of the other drives.
  ■WD Reds are good, affordable and quiet - and because they were built with NAS arrays in mind (lots of hard drives operating in close provimity) - they don't vibrate too much, which in theory is good for the longevity of the drive - especially if you're using a docking station or RAID system that keeps active drives in close proximity of one another. If I remember correctly, the WDxxEFRX range (WD10EFRX, WD20EFRX, etc) use just one platter per terabyte, making them even more reliable than older drives that used more platters. The 1TB WD Red WD10EFRX, for example, just uses a single platter for the entire drive - I would expect this to be a very dependable drive (though you should ALWAYS have your data on more than one drive if it's important to you - all drives fail eventually - guaranteed). The Reds usually come with a 3 year warranty, but the relatively recent WD Red Pro comes with a 5 year warranty. It's more expensive and I don't know how the drive differs, but I may consider using them in the future.
  ■The WD Black is the consumer flagship drive - 5 year warranty as standard. It's very fast, but it runs hot and loud. I used to use WD Blacks almost exclusively, but after experimenting with WD Greens, I then switched exclusively to Reds as my preferred drives for build quality, low noise and affordability.

I've used maybe 35 or so WD drives from the ranges detailed above. I've experienced maybe 3 or 4 failures over 5 or so years - so far only one drive has failed immediately after it arrived (might have been poorly handled in transit - who knows). All failures occured within warranty (and were replaced for no more than the cost of return delivery), but with the exception of the immediate failure, they were all well used.

Drive size depends completely on how large your backup is.
5400 or 7200 depends somewhat on how large it is. But if it runs overnight, it probably does not matter if it takes 2 hours vs 2.5 hours.
USB 3.0 does not matter at all for a regular HDD.

If you are storing critical data, I would seriously, seriously recommend buying two backup drives. Preferably of different models (possibly different manufacturers), and if it REALLY matters, stored in two separate locations in case of fire or theft. This needn't break the bank, but is especially desirable if you are leaving your backup dormant for long periods of time. The particular vulnerability here is that if you make a backup, leave it stored for a year, then actively use your main drive until it fails - you are then entirely dependent on a backup that might not have been tested in 12 months. Passively stored drive failures aren't particularly abundant, but they do happen. If you can't stretch to buying two drives, I would definitely recommend checking your backup drive at least once every couple of months.

As I inferred previously, I don't personally buy externals any more - but I can tell you what I'd look for if I were:-

Capacity
If it's purely going to be used as a mirror image for your existing drive, then it need only be as big as the drive you're mirroring. As you say, you only have ~220GB at the moment, but if you're likely to be using more in the future, you might want to consider getting something that matches the capacity of the entire drive - that way you'll always be able to update the mirror no matter how much you've used up. Going any bigger than your main drive might be a waste if you're not intending to use that extra space (of course, a larger drive might allow you to store multiple mirror images which may or may not be desirable).

Platters
Given the capacities you've been mostly discussing (500GB-1TB), a single-platter drive is usually going to be more dependable because there are fewer moving parts that can fail. The number of platters may not be advertised on most external drives, but by Googling the manufacturer code you may be able to find out without too much trouble. Platters are irrelevant on an SSD, in case there were any confusion.

Reviews
Unless a discernible design or manufacturing fault has been identified, it's very difficult to judge the quality of drives based on just one or two reviews by individuals - each will only have a their personal experience to go on. Unless there is a very high volume of reviews, those accounts are unlikely to be representative of the product over a spread of many thousands of units. For this reason, online stores with the largest quantity of reviews tend to be invaluable in avoiding the worst drives - even if you ultimately choose to purchase from a different supplier. Amazon is often great for a high volume of reviews of mainstream drives.

Warranty
A long warranty is an indication of the trust a manufacturer has in their own product. Couple that with good reviews and a long warranty is a good yardstick. Most external drives only have a 1 year warranty (compared to ~3 years on many internals) - if you see well-reviewed drives with a 2-3 year warranty, this is encouraging. Be aware that the warranty may differ depending on your supplier - Amazon lists 1 year on many drives that come with 3 years from Dabs, for example.

RPM Performance
5,400 and 7,200 are fine if the drive is good (go with reviews and warranty) - and generally, neither will run too hot unless you're working in an oven of a room. 4,200 is a annoyingly slow for a drive you're actively using, in my experience, while 10,000 often comes with performance at the expense of longevity.

Interface
USB3.0 is a terrific interface - in theory. Some motherboard chipsets don't play nicely with USB3.0 (random disconnects and the like), though you're more likely to run into trouble with larger drives - and if you are unlucky enough to experience some grief, you can almost always use a USB2.0 port (at USB2.0 speeds) as a fall-back. So, in short, USB3.0 is fine - and if your computer properly supports it, you'll enjoy fast transfer speeds (and 'typical' speeds if not).

Some drives I would consider (with the caveat that I've never personally used them - and you may need to use a different supplier if you're not in the UK);

• 
  ■WD 1TB My Passport Ultra USB 3.0 2.5" Hard Drive Red - Mfr#:
  WDBZFP0010BRD-EESN (http://www.dabs.com/products/wd-1tb-my-passport-ultra-usb-3-0-2-5--hard-drive-red-8QH5.html)
  ■Transcend StoreJet 25M3 1TB Hard Drive USB 3.0/2.0 - Mfr#:
  TS1TSJ25M3 (http://www.dabs.com/products/transcend-storejet-25m3-1tb-hard-drive-usb-3-0-2-0-9L6W.html)

Hope that's of some use.