I'm having the same issue, on v6.8 spin down was working fine but ever since upgrading I've had an issue,
Drive wake up seams to be every hour,
Noticed quite a few similar bug reports on the forums, most seam to report solving the issue by identifying something accessing the array, however not 100% convinced there isn't a bug.
I'm currently debugging on a clean install of 6.10.3 & this is still an issue