Risky Maneuver ?

OK so I've got a Readynas Ultra 6+ and in there I've got the following

6x Seagate 2TB drives
Model : ST2000DL003

I bought the NAS and the drives back in Sept and since then I've had 3 hard drive failures in the NAS, the first two I sent back to Amazon for replacement (despite me thinking they were actually fine) however my third drive failure which happened 2 nights ago I decided to test, I attached it to my Mac and decided to give it a test with drive genius, after repartitioning it using Disk Utility. I ran a bad sector scan and it found nothing. So I thought what the hell and removed the partition table on the drive and put it back in the NAS, the NAS has now rebuilt the disk again and hey presto everything seems to be working again. I've done a SMART check from within the NAS and I get the following details back

SMART Information for Disk 6

Model:	ST2000DL003-9VT166
Serial:	5YD4J1ZW
Firmware:	CC32
SMART Attribute
Spin Up Time	0
Start Stop Count	494
Reallocated Sector Count	0
Power On Hours	8189
Spin Retry Count	0
Power Cycle Count	16
Runtime Bad Block	0
End-to-End Error	0
Reported Uncorrect	0
Command Timeout	1
High Fly Writes	0
Airflow Temperature Cel	27
G-Sense Error Rate	0
Power-Off Retract Count	9
Load Cycle Count	494
Temperature Celsius	27
Current Pending Sector	0
Offline Uncorrectable	0
UDMA CRC Error Count	0
Head Flying Hours	3612067500819
Total LBAs Written	2206808489
Total LBAs Read	1932480794
ATA Error Count	0

So what gives, is the drive knackered and I'm just on borrowed time, is my Readynas faulty and its reporting perfectly good disks as dead, or is there another explanation, I know I'm getting pretty sick of dead disk reports so what gives ?. All suggestions welcome