Drive mount locations in Android are different to Windows. Android is based on Linux and the Linux file structure is used.
In the pre-installed file manager and the pre-installed HiMedia video player a Windows like file system is emulated. So the built in storage shows C and external drives are D, E etc.
In all other apps the drives use Android file structure.
All directories and locations are subdirectories of root, which is symbolised as /
The internal storage is located at /mnt/sdcard (best for most apps that look for sdcard storage, a relic of Android as a phone OS).
External drives are located at /mnt/sdb/sdb1 then /mnt/sdb/sdb2 etc.
So if you're looking for an external drive in XBMC or another app go to the root filesystem option and look in the /mnt directory!