Hi,
1. Power off the TNAS device, unplug and plug the hard disk , will it recognize the error after turning on the device?
2. Power off the TNAS device, Put HDD2 into HDD1 slot,HDD1 into HDD2 position then turn on the device (or other hard drives). Is the recognition normal?
3.The S.M.A.R.T tools list in the TOS UI is not powerful enough to determind if your hard disk is really healthy.
To have a full scan for your drive, you need some professional tools refer to this link:
viewtopic.php?f=75&t=1125