What have you tried that hasn't worked ? Are you sure this is the class you want for what you want to do ? What exactly are you trying to achieve where you think you need to modify some bytes in the boot sector of a USB flash drive ?
To modify a device at such a level under Unix, you usually require access to the character device special file which will present you the device in "raw" mode. Under OS X, I believe these files will be /dev/rdisk# where # is replaced by the disk number.
NSWorkspace doesn't sound like what you need at all, except maybe to get a list of mounted media using the - (NSArray *)mountedRemovableMedia method. But frankly, this won't give you the DSF for the USB media. openFile doesn't do what you think it does, it basically acts as if you've double clicked a file.
You probably need to look a bit deeper here, the I/O Kit API is probably more in-tune with what you want to do, along with the I/O Registry :
Did you read my second link ? That's a pretty hefty document I pointed to (the I/O kit reference). It should contain everything you need to know. But again I question why you feel the need to even modify the boot sector of a USB drive ?
I did a project like this a few years ago. I ended up reading the /dev/rdisk file. The link below will explain how the data is structured. Open the /dev/rdisk file for reading and march down the bytes.