Have just added a hard drive to my son's xbox360 but it isn't recognising it. what should i do?

If you have formatted your Xbox 360 Hdd on your PC then you may have bricked your Hdd as if you write over the 4KB SS (Security Sector) you can get the drive to function as a Xbox 360 drive again.

Sir i think the way how you added a hard drive on your xbox 360 is not correctly... try this instruction on how to add a hard drive on your xbox360...

The Xbox comes in two versions, one with a hard drive and one without. Enhance game play and speed by adding a hard drive to cache the game data during play. Adding a hard drive or swapping out the original for a larger drive is easy and gives you as much storage as you could ever want.
Difficulty:Moderately Easy Instructions

1. • 1 Select a hard drive with enough space to hold all of the games and media you wish to carry with you. Look for drives with 7200-RPM specifications for faster speed when loading and caching games.
     
   • 2 Format the hard drive to FAT64 using the Windows format utility. Drives larger than 32 gigs will require third party formatting software although Windows 98 is capable of formatting up to 127 gig drives.
     
   • 3 Turn off the power and remove all cables, including the audio, video and controller cables. Lay the Xbox on its side with the front facing you. Look at the left side and pull the hard drive cover away from the console gently.
     
   • 4 Place the hard drive over the Xbox hard drive bay with the tapered end towards the front of the console and the connector end toward the bottom. Connect the cable carefully and verify no pins have bent.
     
   • 5 Align the hard drive slot with the back of the drive and set it into place until you hear a click. Press the cover on carefully and connect all of the cables. Turn the power on and verify the Xbox powers up properly.