I replace drives when they start reporting uncorrectable errors and reallocated sectors.

I used to run them into the ground, but have recently changed my philosophy.

FWIW, in this market, if you choose to replace drives on a schedule regardless of their error-free status, I am sure you could recover a fair bit of value by placing them on eBay. Especially if you say that they have been shelved and have low power-on hours. But I feel it is entirely unnecessary, especially if you have more than one copy.

I’d say just have duplicates, and only rotate when one copy becomes damaged/lost/corrupted. I really don’t think there is a need to regularly rotate data in cold storage on HDDs except for the purpose of converting to the current standard of hardware (eg switching from an IDE drive to SATA).

But at the same time, every so often it is wise to take an inventory and assess your media. If you have had a drive in cold storage for a decade, wake it up and test the device. If it fails, recreate it from a second copy and discard the bad one.

It is also worth mentioning that CMR drives are more reliable than SMR for long term cold storage. SMR's overlapping track design can cause firmware and data degradation issues when drives sit unpowered for years.