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On a HDD I do NOT recommend using the SATA3 ports, but IF they're all SATA3 then verify that the HDD if using with a SATA3 'Interface' that it has the latest FIRMWARE installed. Often the Western Digital SATA3 HDD are problematic on SATA3 ports.
To answer your question, no HDD is faster than 120MB/s-150MB/s or in other words 1/2 the SATA2 'Interface' speed. No, SATA3 HDD 'Interface' is NOT required the SATA3 Interface is fully backwards compatible.
You'll have less issues with a SATA2 HDD and will not suffer any speed loss.