Unless you don't want to use that drive until you run chkdisk on it, it would be a good idea to run chkdisk to rule out a file system error, or to see if it is indeed a hardware error. My bet is a hardware error, which means it would be a good idea to copy any data on that external drive to different media ASAP.
The only difference I can see between the two is the Expansion is a desktop drive (which requires a power pack) and the Backup Plus is a "portable" drive and only requires power via USB. There is also a "portable" version of the Expansion too.I'm obviously not going to buy another WD disk so I guess Seagate is the only option right now. I'm looking at Seagate Expansion and Seagate Backup Plus. Any suggestions?
If you are intending to use either of these drives in Linux, the included software will likely not work.