Some of my oldest NAS HDDs currently in use (Seagate ST3000VN000's, from before they got the IronWolf brand name) have been in continuous use for over 5 years. Thus far they've served very well, but given HDD failure rates tend to exhibit a u-shape, with failure rates starting to rise around 5 years, I'm considering replacing them pre-emptively them with some newer drives. At what point would you start replacing HDDs as a precautionary measure?