Also, if your NAS supports SMB v3 then Roon will enable real-time watching for the storage location. You can disable the real-time watching if you configure the NAS to support only SMB versions lesser than v3. Roon will then revert to a scheduled watch for changes scheme (like eg. every 4 hours). Depending on your sleep time settings for the NAS and the check schedule in Roon, this will probably allow the disk to got to sleep for a couple of hours before they wake up again.
Note: This may allow the disks to go to sleep but may also lead to other problems (depending on how long the disks need to resume work, the storage may be marked as unavailable or backups may fail). Modern disks designed for the use in NAS devices, are meant to run 24x7. It’s best to disable sleep mode on your NAS.