Do you want a desktop, portable or NAS backup drive? If you'll need to take data on the go, consider a portable drive that doesn't require an external power supply. While these generally hold less data than desktop drives, most still offer enough capacity (1 TB or 2 TB) for most personal storage needs. Keep in mind that portable drives, because they pull power solely from the USB port, can drain a computer's battery faster than a desktop drive.
Those who plan to backup large multimedia files will be better served by a desktop drive with higher storage capacities as well as faster data transfer speeds. For people who or businesses that want to backup multiple computers or access files on a shared network, consider a Network-Attached Storage (NAS) device.
Do you have a need for speed? Data transfer speed, measured in megabits per second (Mbps), is almost always a major factor in reviewer ratings -- though this is only a big deal if you use the drive actively, and it might not matter at all if the drive is just for automatic backups. The type of storage technology (spinning platter or solid state) plays a factor in this, but other factors such as interface type and buffer can be more crucial.
As an example, the large RAM cache of the Buffalo DriveStation DDR helps that USB 3.0 drive beat out many Thunderbolt drives in data transfer speed tests. Of course, if you're using a not-so fast connection type like USB 2.0, that itself will create the bottleneck, not the drive itself.
What is your backup strategy? Experts say that every external hard drive -- no matter how reliable -- will eventually fail. Because of this inevitability, a hard drive storing irreplaceable data should be a part of an overall backup plan – not its entirety. Experts recommend creating two backups; storing identical data on two different drives virtually eliminates the chance you could lose all your data following a drive failure. Some backup devices contain multiple drives, which can be set up in a RAID (redundant array of independent disks) configuration to mirror the drives, so if one should fail, the other drive's data will still be safe.