It's 12v by the way. I've tested with the WD Green & Seagate ST4000VM 4TB drives the Seagate runs about 28 degrees constantly and the WD 33 degrees. These temperatures are apparently near perfect or probably the same as in your pc
The fan connects to the direct main feed (i.e. the one to the disk) not the one for the back fan. I've left the back one & it will come on if the temp happened to get high.
It is a bit drastic so you need to be careful - the top of the plastic enclosure is very brittle so do it bit by bit after...
Yes I have custom firmware and really this does away with the small Humax fan although it will kick in at 50 degrees. I've checked again and the average drop in temp is 20 degrees using Sysmon the temp has literally dropped off a cliff!
I'll put photos on when possible
I've been getting the message 'too hot' on the front panel for some time now often the disk will be over 50 degrees. Even under normal operation the disk runs very hot so I decided to put some cooling in.
I have bought a hard disk heat sink & fan which 'just' fits on top of the disk after...