As it stands, this isn't a Computer Science question. Actually, it's several questions, some of which could be phrased of as computer science questions.
Disk-like devices (these days we'd include flash memory) do not understand the concept of a "file" or "directory". You can think of a disk as a big array of fixed-size blocks. The two basic operations are "read a block" and "write a block". Files are an abstraction on top of this implemented by the operating system.
All of this is explained in any operating systems course. But if you just want the short version, I recommend Maurice Bach's classic book The Design of the UNIX Operating System. It doesn't reflect modern operating systems or filesystems that well, but it will give you a good idea about what operating systems do and what they expect of a disk device. If that wasn't enough for you, Practical File System Design by Dominic Giampaolo is a whole book which gives an in-depth look at a more modern filesystem design.
The hardware that sits between there and the disk is a topic all by itself. On a modern system, disks can be connected to a computer using a variety of different hardware connection (e.g. USB, SATA, SCSI), and a modern operating system needs to both be able to talk to the host controller for this bus, but also be able to tell what is attached to the bus. For some busses (e.g. USB) there may be a complex topology for the operating system to discover; the disk may be connected via a hub, for example.
The final link in the chain is what lies between the bus and the storage medium itself, and this is presumably what you're interested in.
My suggestion, if hardware engineering is what you're most interested in, is to hook a simple computer (e.g. a microcontroller) up to a simple storage device (e.g. MMC) and see what you can do. You will learn a lot this way.
As a final comment, there is a book by Jan Axelson called USB Mass Storage: Designing and Programming Devices and Embedded Hosts. I can't recommend it because I haven't read it, but it looks like the sort of thing you might want to know. If I'm reading the table of contents correctly, it looks like it gives you everything you need to know to build a USB flash drive. That sounds like a fun project to me.
