When the MagicJack USB device is inserted, two drives are created. One is a pseudo CD-Rom drive (read only). The other is a writable flash drive. In some cases, it may be necessary to force these drives to use other drive letters. This can be accomplished with the following instructions.
Start the Windows Computer Management tool by right clicking the "My Computer" icon on your desktop, and choose "Manage."
Alternatively, this tool can be found in your Start->Settings->Control Panel->Administrative Tools.
Navigate to Storage -> Disk Management. It may take a few seconds for it to query your system.
In the list you will see a drive labeled "PHONE" and a drive labeled "magicjack."
Right click on the drive labeled "PHONE" and in the subsequent pop-up window select "Change drive letter and paths."
Click on the 'Change' button, select an available drive letter from the drop-down list, and click the "Ok."
Repeat this process for the drive labeled "magicjack."Remove the MagicJack device (using "Safely remove hardware), and plug it back it. It will start up using the new drive letters that you have assigned.
Credit: Taken from http://www.phoneservicesupport.com/change-mj-drive-letters-t4169.html