I assume you logged into the NAS webUI, went to 'Management' --> 'Disk Management' --> 'S.M.A.R.T. Test' and ran the quick smart test and it shows 'failed' for one of the disks and you now want to know what actually failed. I also assume the NAS is not reporting a degraded array.
There are quite a number of SMART attributes but only a small number of them are critical and indicate the disk should or must be replaced.
Unfortunately, the NAS does not give a full report, just a pass or fail.
To get a full report you need a few things and some preparation.
1) download the disk manufacturer's disk tools (get a bootable cd iso if possible, especially if your computer is using windows and burn it to a CD)
2) get a SATA USB adapter/caddy (must be able to power 3.5 inch drives)
Now you can power off your NAS, extract the driver you want to test (indicated by the webUI). Attach or insert it in the adapter/caddy.
Now you boot from the CD and attach the USB mounted disk. Once you have indicated the disk to the program just follow the instructions on screen .
If the disk tests end in a satisfactory manner you can replace the drive in the slot you took it from in the NAS box and power up the NAS.