Inside the BPN-DE340SS, there is one PCB that handles the electronic functionality of the drive cage. One side of the PCB has four SATA power and data connector pairs for the hard drives to connect to. There are nine screws holding down the PCB to the cage, which is important for a device like this since you'd want the part hard drives are being connected to and removed from to be able to handle many usage cycles solidly.
The other side of the PCB faces the back of the cage and the interior of the case the cage is installed in. Each drive has its own SATA data connector. Looking at the traces, we can see that the SATA power connectors provides power for two drives each (with one connector also powering the cooling fan).
The cooling fan is an 80x80x20mm fan from the Young Lin Tech Company. It's a sleeve bearing fan rated with an airflow of 32CFM with 37dB(A) of noise while running at full speed. While the physical aspects of the fan itself are universal with most other 80x80x20mm fans, the connector isn't. It still should be an easy task to switch fans.