Do you have another device using J: when the letter changes, like a flash drive? Also, when you remove the external drive, do you safely remove the hardware by using the safely remove hardware icon on the taskbar? You should do that if you want to keep drive letters necessarily static. The reason is that safely removing hardware lets the OS know that the device is no longer present - without doing it Windows has no way of knowing is the device has simply become unresponsive.

If there isn't another device using J as its drive letter and you always safely unmount the drive, try this command from a command prompt (Start > Run > "cmd"):
Code:
subst j: h:\