PCDOS success with SATA depends on whether the bios offers any IDE compatibility settings. Have you checked if this option is available on your 7520?
More and more vendors are not supporting PCDOS so what works for older hardware will stop working with new hardware, and sooner or later you will have to start using WinPE. That is what Microsoft have been using for Vista Windows 7 and Windows 8 as part of their automated installation processes, and there are valid technical reasons for this.
PCDOS also lacks support for USB devices - how are you connecting external drives for backup currently?