Hi vincenlolo,

Thanks for posting.

Your post doesn't indicate which OS you are using.  Your Latitude E5420 supports these operating systems:
Windows XP
FreeDOS
Windows 8, 64-bit
Windows Vista, 64-bit
Windows XP, 64-bit
Windows 7, 32-bit
BIOS
Windows 7, 64-bit
Ubuntu 10.10
Windows 8, 32-bit
Windows Vista, 32-bit

Here is a link to the support page so if you are using one of the above OS.  You may wish to reload the docking profile drivers.  http://dell.to/2wDooeY