Western Digital, Seagate, Toshiba and Buffalo Technology are all well regarded brands, I've owned at least one drive from each without any complaint.
Decide on the size/capacity and then look for the best value IMO.
1.
Decide on which physical size (2.5" portable drive or 3.5" desktop drive- the former are physically smaller and usually powered through the USB connection, whilst the latter tend to be cheaper/TB, have faster disc speeds and use a separate power adapter).
2.
Choose a capacity. 2TB seem to currently offer the best value for 3.5" drives, typically only £15 more than their 1TB versions.
3. Many drives are
USB3 capable, but not all. If your computer is USB3 capable (most are) then it offers significantly faster data transfer (5-10x compared to USB2) which reduces waiting times.
4. Check
what's included with the drive: USB cable? Is it long enough for your intended use? If not buy online to save being ripped off by the high street chains who grossly overcharge for cables. (To get the full speed benefit of USB3 you need to use a USB3 cable, they differ from the older USB2 versions).
The linked thread bemoans, rightly IMO, the lack of power switches on most external drives these days. Having had a power supply fail and destroy a hard disk it is wise to disconnect your back-up/storage drives when not in use.
One simple solution is to use an
in line switch like this for the 3.5" desktop type drives.
I've used
Ebuyer and
Overclockers for online PC parts, both have been reliable suppliers, although even Argos and Tesco occassionally have good deals so may be worth checking. HTH
[In an ideal world we all follow the
3-2-1 strategy for data storage and backup, just something to consider before committing all you photos to a single drive].