Hard drives should not be left unattended for even a few years with any expectation that they will still function when plugged back in. Sure, many will, but it's a crap shoot: Under ideal conditions an unused hard drive can be expected to retain data for 9 to 20 years. [0]
Well yes but I heard that b4. You should just put on your calendar to plug them in (and update the backup) once in a while. I do not think it is about mechanical robustness. It is about the way the data is stored with magnetic tech, it think HDDs should be operated every once in a while to have a better chance of not loosing the data. But 9 days sounds awfully short.
I found the approximate time frame in a few places, but never with any primary source. I also came across the claim that hard drives used to be more mechanically robust, and perhaps failed less often for such reasons. Again, I couldn't source it though.
[0] https://blog.macsales.com/43702-we-bet-you-didnt-know-that-y...