'Minix 3', and 'Usbdlm'

Minix 3

MINIX 3 is usable as a serious operating system on resource-limited and embedded computers and for applications requiring high reliability. This new OS is extremely small, with the part that runs in kernel mode under 4000 lines of executable code. MINIX 3 is targeted at the following areas: Single-chip, low-RAM, low-power, $100 laptops for Third-World children, Embedded systems (e.g., cameras, DVD recorders, cell phones), applications where the GPL is too restrictive (MINIX 3 uses a BSD-type license), and education (e.g., operating systems courses at universities).

http://www.minix3.org/

Usbdlm

USBDLM is a Windows service that gives control over Window's drive letter assignment for USB drives. Running as service makes it independent of the logged on user's privileges, so there is no need to give the users the privilege to change drive letters. It automatically solves conflicts between USB drives and network drives. Furthermore you can define new default letters for USB drives and much more.

http://www.uwe-sieber.de/usbdlm_e.html