Which storage device to buy?

WD External Drives External Drives for Mac
2

If you have a thunderbolt connection on your computer, you might want to take a look at the WD Thunderbolt Duo

This unit goes up to 8 TB and is RAID 0 and 1.

Regarding the EX4, if you are going to be doing an automatic backup to this drive, make sure that the software that you have is capable of doing a network backup since the EX4 needs to be connected to your router.