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On the front bottom of the case, as stated formerly, you can see 3 3.5" slots for HDD storage.
Three 3.5 internal bays, a suitable space for hard disks, that are properly cooled by the fan and easy to install. Not a screwless design though, let me show you.
Basically you slide in the HD and if you do not plan to move the PC it's pretty secure already without screws.
Yet I recommend you secure the driver. And this is tricky, you'll need a screwdriver with a magnetic head.
Wheels, they need to be screwed to the casing. A little Spartan I know, but hey. This case is so heavy, you're gonna need them. The wheels have brakes btw.