I found that the HD204UI disks get the Advance Power Management setting of 224:

hdparm -I /dev/sdb | grep Advanced
Advanced power management level: 224
* Advanced Power Management feature set

The problem with this setting when you use (tested) at least RAID5 or 6 the disks will get a Load Cycle Count every 30 seconds.

The Synology or controller polls all the disks every 30 seconds (you see the lights blink from the first disk to the last one in the array).

This results in immense Load Cycle Counts (this disk is 3 weeks old and already has 30,000 load cycles):
smartctl -a -d sat /dev/sdb | grep Load_Cycle_Count
225 Load_Cycle_Count 0x0032 097 097 000 Old_age Always - 30524

If you set the advanced powermanagement to 254 or 255 (which is less green offcource) this problem is solved.

Please investigate this and patch it in future versions, because at this rate the disks will break after ~1,5 - 2 years (600.000+ total cycles) and the clicking noise is terrible to hear from a harddisk

Thanks for your time.

Regards,

PS, i tested this on 2x DS1812+ of my own, and also with a DS1812+ of a friend of mine who has the same disk type.