Internally this system case is designed in such a way that it doesn’t have 3.5” bays at all. To install devices of that form-factor you should use two special cages which are put into the 5.25” bays and one more cage which is located at the top of the case, near the power supply. The first and smallest cage is for floppy drives, card-readers and such:
The 3.5” cages and all 5.25” devices are fastened with plastic locks that fit into the screw holes in the drive’s sides thus making the whole installation process as easy as two actions – insert the device and close the lock. I must confess this is one of the best solutions I’ve ever seen. You don’t have to screw anything to the drive and there’s no need for rails and fasteners.
The cage was not made properly. There is no external faceplate for this basket, so if you want to use a floppy drive, you will have a hole instead of one faceplate, with the floppy hanging in the air [Надеюсь, правильно перевел, хотя я не очень себе представил, что значит «для нее нет внешней заглушки» и что именно будет при установке флоппика]. On the other hand, it is rather an exceptional situation today for a modern system to have a floppy drive. As for card-readers, you can purchase an external one, so this is hardly a serious drawback.
The second cage in the 5.25 rack is going to accommodate your hard disk drives.
You should release three locks to extract this cage out of the case.