Thermaltake Aquabay M2 5.25" Bay Reservoir
 

As you can plainly see, even though the bezel is 1U configuration, (1U = 1-5.25" bay) the fan means you'll need at least two free bays. And if it is going to work well, you'll need clear space both above and below the radiator for sufficient air movement and above average case ventilation. So in reality, the M2 Radiator will need 3 or even 4 clear drive bays to function. A shallow bay device such as many of those in the Hardcano line, which only extend a short distance inward past the front bezel, would not present a problem mounted directly above the M2 AquaBay.

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