So currently I run two NASs:

NAS1:
24x HGST HUS724040ALS640 SAS HDDs
FreeNAS 11.1-U2

NAS2:
Windows 7 w/ Drive Bender
20x WD HDDs (various types and capacities between 1TB and 3TB, mostly Reds and Blacks and 1 or 2 Greens)

Now I always knew that best practice was when having a backup NAS to have a different OS in case any updates or bugs affected your primary, they wouldn't affect the backup. But does the same hold true for HDDs? The HGSTs that I use are pretty darn reliable and have had no issues with them, so I wouldn't mind eventually replacing all the various ones in my backup with those if that's the consensus. But on the other hand, if there's some sort of inherit flaw with those drives, I would be infecting both NASs. Mind you, I have 60TB of stuff on my primary so reliability is of great concern (one reason why I went with HGST/WD).