Question: I use git bash under windowsXP system.I try to enter my USB Disk by entering "cd G:" in git bash,but it apears that "sh.exe : cd : g:: No such file or directory".

I can open my USB Disk under my system.

What's wrong with it ?How can i solve the problem?

Answer: 
Git bash uses the `/x` form to identify the root of drives, instead of `X:`. Doing `cd /g` should work correctly (you'll likely see similar paths while working within it). 

I believe this is just how bash handles the drives on Windows (it is used through all the versions of git bash I've used, on XP through 7).
If you don't want to close all bash windows, or that doesn't help, you can share the drive, and access the network share. Worked for me, usb stick at `//localhost/usb` works fine, closing bash didn't.