ST31000340AS and ST31000340NS are different model. The NS model is intended for 24x7x365 use with heavy I/O. The AS model is intended for light usage but should be fine for use with the DNS as well.
Check the firmware revision on your HDD's and make sure they are at the latest firmware level. Seagate had a period where their HDD's were experiencing numerous problems that were resolved by updating the HDD firmware.
If you are at the latest firmware level, it is likely that the HDD in question is failing. Download the Seagate HDD diagnostic tools and verify the HDD.
HTH,