It's simple. You pick up a Noctua fan with the low noise adapter, and then this cable (or whatever alternative. I use this one specifically). This supports two fans, of course, and you plug this into a dual port USB charger if using two Thunderbays or a computer etc. All you're doing is installing the Noctua fan in the exact place the original was, and then running the cable outside the case to this USB cable:
Here's the cable:
The CRJ USB to Dual Fan Power Adapter Cable stands out as the first full-speed USB adapter designed to power computer fans, offering unparalleled flexibility in utilizing almost any fan, PWM controller, or compatible device. Equipped with a high-efficiency voltage step-up module, this cable deliv...
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And here's the fan with the low noise adapters:
Featuring advanced aerodynamic design measures such as Flow Acceleration Channels and Noctua’s AAO frame, the NF-A6x25 FLX is a highly optimised, premium quality quiet fan in 60x25mm size. Smooth Commutation Drive technology and Noctua’s reference class SSO2 bearings guarantee superb running smoo...
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You remove the fan grille on the back of the mini (just 4 screws as I recall) so that the cable can come out through it. Then you put the grille back on. The whole process of installing should only take a couple of minutes