Instead of buying an external drive, but an external enclosure then put your own drive in it yourself, its really easy and then you know what your getting.
I upgrade the drive in my laptop every so often when the price of one twice the size of the one thats in it comes down to the same so I put the previous drive in the enclosure for a backup.
Now don't stop with an external drive, your violating a rule of thumb for backups by having the backup be on the same media type as the original, you want one or more backups on some sort of different media and in a different location. You may laugh at that but when your pictures start to number in the thousands it becomes important to follow all the best practices.