Stupid question

I dont' think you can put a writer into an INTERNAL bay as you can't get the disk in or out. Thankfully I don't think to many cases come with such a bay. If by chance you meant the external bays then the first thing that comes to mind are the card readers. While many come in 3.5" size, some are 5.25. There are also fan controllers, high end harddrive cages, sound card front access "adapters", and many other things. (I even saw something that gives you two tiny 2W speakers.)
 

I did mean external 😛 and thanks