Is there any way to read / write to a USB Memory Stick using INT 13? I'm not sure what DL should be - < 80h is diskette, >= 80h is HDD right? So what is USB?

If there is another way, I'd love to know ;)

Unlikely unless the BIOS provides some support for faking it being a 'normal' harddrive (similar to what most can do with USB keyboard & mice).

I think I've heard that some BIOSes can boot from an USB drive. That would mean it has to be accessible through INT 13. It might only work if you're actually booting from it though.

Both laptops that I've had have been able to boot from my USB floppy.  Don't know how it's done, though.