I got my first computer (that was truly mine), in the summer of 1996. I've had probably 20 hard drives over the course of time from then until now, and none had ever failed. Some even lasted for 8-10 years before I retired them due to speed/size. And, well, it still might be OK...but I'm not risking it.
A few weeks ago, I noticed one of my drives not passing the SMART diagnostic at boot, but after error checking, nothing wrong was found. Then yesterday, I noticed movies playing off this drive were stuttering, which should NOT happen on my machine. I had a feeling the drive was going, so I started copying the few directories on the drive I hadn't backed up recently, and my transfer speeds were...get this...500 KB/sec. I started backing up a 68GB folder with a bunch of movies to another hard drive this morning. When I got home from work, it was 50% done. A check with HD Tune showed reallocated sectors.
So, I bought a new 2TB drive today (the one with issues was 500GB), along with a USB3 enclosure (had been wanting one for a while), and swapped them out. I also replaced the SATA cable at the same time, just in case that was the culprit.
Well, with the 'failing' drive in my enclosure, I'm copying the files to the new hard drive, and now, I'm getting 120 MB/sec transfer.
So, I guess it's probable it was the cable...but I don't really trust the drive any more. And hey...I needed some extra space anyway.