Choosing the right Device Class for your application

USB Mass Storage Device Class (MSD)
[Or] The "Flash Drive, MP3 Player, External USB Hard Drive, Digital Camera" Class

Two books that covers MSD to the core are USB Complete and USB Mass Storage .

2. Benefits of using MSD device in your application

Mass Storage Devices are almost supported by all existing operating systems. For Windows 95, the MSD device used to come with it's own Windows 95 driver, as Microsoft did not produce any Generice USB MSD driver by then. Similar case with Windows 98 too. Almost all  versions of Microsoft Windows starting from Windows 98 SE, and all most all Linux variants, latest MaxOS versions give good support to Mass Storage Device Class. What it means to you as an USB device developer is, you don't have put effort on developing OS specific device drivers. These device drivers are already available for your use.  The only catch is when you design and develop a Mass Storage Device hardware/firmware you are essentially developing a device that will be viewed by the host computer as a Disk Drive. And that is what you need to remember.  

Windows Mobile supports accessing of the most USB mass storage devices formatted with FAT on devices with USB Host. Also note that there is no native support for Mass Storage Devices in Windows NT.

3. How to interact with any MSD device on Windows

The USB mass storage specification does not require any particular file system to be used on MSD devices. Instead, it provides a simple interface to read and write sectors of data—much like the low-level interface used to access any hard drive—using the "SCSI transparent command set." Operating systems may treat the USB drive like a hard drive, and can format it with any file system they like.

But if you want to provide additional File System support then, MSD device need to implement a File Syetem on top of the firmware. Some standard file systems are NTFS, FAT, FAT32 etc..