Western Digital Elements SE Portable 750 GB is my preferred NAS backup device and I got two of them. They are both a part of my data backup strategy. The Western Digital external disks are extremely popular and it is understandable.
I do like them a lot because of their stability, capacity and connectivity.
Even though my Synology DS211+ NAS has a capacity of 2 TB it does not mean that all 2 TB are used and far from all data is that critical.
The critical data on my NAS has a current capacity that says 300 GB and that data is backed up on the two WD Elements SE 750 GB disks every week. I do make an encrypted image off the content on my MacBook Pro and put it on each disk too. So each disk now contains NAS data and MacBook Pro data.
Each disks is placed on a separate location outside my house and the data is off course encrypted. I do not store my data externally unless the data is encrypted.
I do recommend the Western Digitals Elements series and the My Passport series for backup. Both series are worth the money.