R7800 External USB hdd's that work?

Just bought an R7800.  I do not have an external hard drive, but want to find one that has been proven to work before I spend the $$.  I searched for some of the WD models (prefer WD but will consider others)  from the "tested" list.  Only found 1 model in stock and it cost more  than I expected and was an older model.  Several said they were replaced with a newer model. 

Hope to find actual models that R7800 owners are using sucessfully.  4Tb or more desired.  Anyone successful? Thanks.