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To install the storage units you need to remove the front side panel. You then push the lever towards you and the tray can now be removed for installation of an HDD or SSD. The unit btw is hot-swappable. So if you setup a RAID array, and one drive fails you can hot-swap the drive and exchange it.

 
If we remove all drive caddies, located at the left side you can see the actual motherboard slash PCB. Nothing much is going on there aside from the fact you can see a heat-sink that cools the processor. It's clearly passive and thus makes use of the cooling fan housed in the back of the NAS.
  

in the rear of the NAS we see the SATA3 connectors, four of them. If you are going to install HDDs, please check the ASUSTOR QVL list of supported units. Most of them will all work fine up-to 12 TB. But in the past, I installed four really old HDDs, and when the NAS dropped out of sleep mode I heard all kinds of weird noises. Two of them regained to active status and one was stuck somehow between hot/sleep. Not ASUSTOT's fault of course, but this is why you always need to check the QVL list for supported storage units. 

The NAS rests on rubber feet which will help against resonating noises. The NAS itself is reasonably silent. For our testing, we are mounting a three SSDs in a RAID5 setup in an effort to pass SATA3 speeds on that 10 Gbps connection.

 
So, as stated we received a four HDD tray version of this NAS unit. The NAS supports hot-swappable hard disks and comes with proper disk tray locks. These have been upgraded btw, no need for screws, just pup out the side retention clip, seat the HDD, put it back in and your tool free installation is done.
  

ASUSTOR has an advanced and easy to use tray system, actually very easy to use. You can mount your HDD, secure it and slide it into the tray. By pushing the handle inwards the HDD is mounted and locked securely.