Looking inside…
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In the top left you will see the area where the PSU resides; well you would if it wasn’t for even more drive storage space.
The reason for that 90mm fan alongside the PSU is a simple 3 x 3.5” caddy that is screwed into the chassis and held in place at the top using a spring-clip.
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With the caddy removed you can see the 90mm fan from the inside, along with the space where the PSU resides.