I have an 2009 iMac (11,1) with a SSD (SATA 3) for the internal drive and use a firewire 800 external drive from OWC for my Time Machine back-ups exclusively. I need to change out the external HDD (sadly the top limit is 2TB) and wanted to know if going to a 5200 RPM vs 7200 RPM would cause a bottleneck. My choices for replacement HDD are: $95 WD Red 2TB (5200 RPM) (WD20EFRX) $132 WD Black 2TB (7200 RPM) (WD2003FZEX) $149 WD RE 2TB (7200 RPM) (WD2000FYYZ) If I don't need to pay for 7200 RPM why spend the extra bucks. But reliability and durability are my primary concerns. Any advice is appreciated!!