I recommend buying the external USB enclosure and a bare drive separately so that if the drive ever fails you won't be stuck with a proprietary device that will only accept USB connections. I have had two WD USB drives fail on me in the last 4 months, so be warned. Some of these compact USB drives do not have standard SATA/PATA interfaces (internally) which makes trouble shooting and recovery virtually impossible.
I concur with canadian69; I prefer buying a separate HDD and enclosure. Generally Not as pretty.
You mention “reliability and Not performance – Great as Reliability is what you want in a backup drive.
The 2.5” drives seems less susceptible to self destruct when “Bumped” than the 3.5” drives. But over 1 TB the 2.5” drives are very limited. With 2.5” drives the 750 Gig drives appear slightly more reliable than the 1 gig drives.
Looking at the 2.5” drives. Example of drive you DO NOT want;
1 TB WD Blue - 1 & 2 egg (Neg) reviews are a whopping 30 %.
Samsung 1 TB M8 with a whopping 34 % ½ egg % http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Verses the 750 Gig WD Black with only 14% 1 / 2 egg reviews http://www.newegg.com/Product/Product.aspx?Item=N82E168...
And the 750 Gig Seagate Hybrid with 12% negative reviews. BUT with this drive you would be paying extra for the internal 8 Gig SSD. Concept is great for an OS + program drive but would provide NO benefit for a Back up external drive. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Added: The topic of the 1 TB Extrenal drives came up awhile back. When looking at Newegg back then there were a Number of extrenal 2 1/2 in drives with high Number of reviews, mostly negative - THEY seem to have dissapeared as the Nr of raters of these drives is VERY low now!!!!
Personnally I try to keep 2 Backups, But one is OK, it's doughtfull both internal data drive and external drive would fail at the same time.
I'd probably go with one or Two of the 750 Gig WD Blacks and the enclosure I linked to.
One other advantage of the 2.5 over 3.5 drives is portability, But I do use Both.
My Backup external drives are never connected and powered on EXCEPT when I do the Backup.
You could go with the Samsung F3 1 TB drive which is a Sata II HDD with good stats . The 1 TB 3.5 In WD SATA III black is also a very good alternative - just stay away from WD green and Blue drives). Need a 3 1/2 inch enclosure (USB3). I also use this combo. I prefer laying the drive down verses installing it upright as less chance of tipping the drive and killing it - I've lost to external 3 1/2 in drives because the accidently got knocked over.