Pictured below is what is generally referred to as a 'parallel port 3.5" disk drive'. As the name suggests, it plugs into the PC's parallel port. These appear periodically on eBay.
To work, they require a driver. In DOS, that driver is typically loaded via CONFIG.SYS
Booting from these types of drives is not possible.
The particular drive pictured below is a Backpack model 014350, made by Micro Solutions. The DOS driver for it is designed so that the Backpack 014350 can even be used with 8088 based PCs (such as the IBM PC and IBM XT). Click here to see the flyer.