Complex sentence: Like the Series 3, to upgrade beyond internal storage the new units require use of "My DVR Expander" from Western Digital, unlike many DVRs that allow external drives from a wide range of manufacturers.
Split into simpler sentences: Like the Series 3, upgraded storage is only possible with the external hard drive "My DVR Expander" from Western Digital. This is unlike many DVRs that support external drives from a wide range of manufacturers.