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: 
Gina, the real answer is in your comment above. You have to close down not just any Git bash window, but ALL git bash windows and then re-open one of them. At that point, `cd /f` will work. By the way, `cd F:` also works, as the bash terminal converts that to /f. I just confirmed this on Windows 7 64-bit.
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.