When you replaced the hard drives, did you replace the system for a new one with a new ram model or did you replace the ram model in your old ps3 system with a new ram model?
Anyways, you should probably look and see exactly how much RPM the drive has. You might be mistaken. Last i was told, the PS3 has issues with drives with more than 6000 RPM. While nothing will fail, the drive will tend to get hot, causing overheating.