One flashing red light on player 3 and 4 alternate flashing E79

That may be an error caused by the harddrive not making good contact with the xbox, could be external or internal problem. IF YOU HAVE WARRANTY DO NOT OPEN THE XBOX OR THAT WILL VOID YOUR WARRANTY
Try this:
remove that hard drive and try to play without it to see if that's tha problem. if it works put it back on and try it again. If thats not the problem try this

Console reset codes
Clear All Installed Game Updates and Console Cache

1. Go to the "system" blade
2. Select "memory"
3. Press Y on the HD symbol
4. Press X,X, Left Bumper, Right Bumper, X,X
5. A message will appear saying: "Do you want to perform maintenance on your Xbox 360 storage devices?"
6. Select Yes

Clear Any Failed system updates that cause the console to error.

1. With the console off, press and hold the sync up button for 5 seconds (the small white one)
2. While holding the sync button press the power button to turn on the console (do not relese the button)
3. Continue to hold the sync button until the Console has booted up completely.
4. During the boot process the console should clear any failed updates, allowing you to use it normally.

E79 is a hard drive problem. It could be a problem with the Hard Drive itself or a problem with the internal connection to the hard drive, Try removing the hard drive and playing without it