Depends on your space needs and your preferred price range.
I can give you a suggestion and my feedback. I've had good use with my Seagate 320GB that I got given for doing virus removal for friend.
Had it for a few years now and this particular one doesn't seem to do so good on 64bit Windows 7.
Every now and then I would have to disconnect and re-connect for it to work,
(Worked great on my old 32bit PC)
At the time I only had a dogdy 4GB USB and I needed alot more space then that, And at the time my PC HDD was only 30GB lol (Thats what you get for renting ex-rental PC's)... It was basically full so I needed EXHDD.
I've recently just purchased a Seagate GoFlex 1TB since my 320GB was starting to become full and the issue I was having with it,
This GoFlex has the Windows 7 Compatible sticker on it unlike the other one and so far its all working well now on 64-bit.
I had options to buy 2TB ones but I notice there bigger and bulky and require AC Power Adapters to be used, Since my 320 is small and is USB powered I like that Plug and Go feeling since I took it travel alot.
The GoFlex is basically the same in looks but with the gloss finish and is USB powered.
Your either a AC Adapter guy or USB cable so keep that in mind, If your EXHDD is not going anywhere and just going to sit beside your PC then go with AC if you need too.
2TB is overkill in space for me since my new PC has 1TB in built which I still have to use.
The GoFlex caught my attention because it has a feature to change and upgrade cables, Different cable kits such as USB 2.0, USB 3.0, eSATA etc, The factory GoFlex I got came with 2.0 but in time when I get a new PC with USB 3.0, I will then purchase a 3.0 Cable kit.
The speeds I get on my GoFlex is 35 - 43MBs compared to my 320GB 18 - 26MBs.
Until my GoFlex becomes close to full I will purchase another EXHDD in the future.
None of my important data or files is stored on my PC's HDD, I'd rather using my GoFlex, Only stuff kept on my PC's HDD is program files and Windows it self.
Explains why I don't mind reformatting if the needs ever come because its no hassle to me.
Here's a pic of mine and some advertisements from the cheapest Tech store in town and most reputable.
Yeah for abit more $ I could have got 2TB but as I said that's overkill for me and I don't like the large design with annoying AC Adapters.
Before buying my second EXHDD I did research reviews on different EXHDD's from lots of different places online. I suggest you do so aswell and in the end depending on your piggy bank and size needed get the EXHDD you need.
WD from what I see has alot of negative feedback, So that kept me from getting any WD product. Seagate has some bad reviews too but no where near as much WD does, but nothing is perfect right?
GoFlex is future proof thanks to its cable kits., Anyways hope this feedback/mini review helps out. Cheers.