I have bought both an internal hard drive and an external (usb connected ) hard drive.
The internal drive was a 160gb unit from a seller in the UK. I knew that it was second hand but did not realise just how much.
It failed totally after about 18 months service and an IT man checked it out and found that it had done thousands of hours of service. He thought it had probably come from a scrapped server; in other words a 24 -7 job working flat out.
The external -usb connected drive was a 30 gb unit from a seller in Hong Kong. When it arrived, and was checked ( I had assumed it was all-new ) the disk had a virus on it. It was cleaned up but failed after 5 months service. Again, the IT man thought the enclosure was probably new but the disk was probably salvaged from an old ( and infected) lap-top .
I have decied to buy these units from reputable UK sellers in future and stick to factory-sealed.