Bad sectors reported but not found

I have a ReadyNAS NV which has been faithful and terrific for years. I have even upgraded the original disks to Seagate 1TB drives and they have run more or less without fail. Recently ReadyNAS has been reporting a growing number of sector errors on a drive. However, when I put the drive through external testing, both with Seagate tools and Spinrite, the drive always reports as working just fine and no bad sectors are detected. I've rebooted and run volume scans and they never report any errors. I'm running the most current firmware.

:( So, what am I to believe? Is it possible something is failing on the device itself and it is reporting errors that aren't really drive errors? Is there anything else I can do on board to test?