If Sync prevents HDD from sleeping on your NAS, you should try the following:
1) Increase some activity intervals
- Set folder_rescan_interval, config_refresh_interval, config_save_interval to some significant value, for example, 18000 (1 stands for 1 second so 18000 = 5 hours).
folder_rescan_interval sets how often Sync rescans folder for files' changes.
config_refresh_interval controls how often Sync checks for the info about available trackers and relay servers
config_save_interval controls how often settings are saved to storage
2) Disable Sync logging
Sync logging may also be a factor preventing your NAS from sleeping. Try disabling logging in Sync Preferences -> Advanced, uncheck logging option and restart Sync.
Note that if some nodes are updating files or requesting some data from your NAS - this is going to awake HDD.