Oh... that was warranty information -- service life is even more all over the place. Most folks I know who track it are tracking specific LOTS of drives they buy in bulk. They typically fail in similar timeframes, even same drive, same everything... except different timeframe of purchase.
At least one government agency near me here is replacing all disks on a rotating three year schedule, which to me, is excessive.
We have everything from 10 year old (yeah, really... easy in fact) drives in Production to brand new ones fail.
The U-shaped curve of "infant mortality" where more new drives fail than middle-aged ones, and then old ones start failing again on the upper right side of the "U", always applies.
You mentioned these were secondary drives... added to the machines? Proper airflow/cooling? Heat kills drives more than anything. I have stuff spinning that's well past 10 years old... they're in temperature controlled environments, and they're rarely shut down. As long as the bearings hold out, they'll be there.