Hard disk not displaying on desktop?

Here's a question for work? We've got roaming accounts for our macs, but we get issues sometimes when students log in that the icon for the hard disk doesnt appeard on the desktop or in the finder....

Weirdly related to this is when this happens neither firefox or safari can load any webpages...... However, if students log out and log back in again the internet works fine.

Im assuming when the hard disk isnt available, then firefox and safari cant access its preferences and necessary files properly, but what would be causing the hard drives not to mount/be visibile?

ANy help greatly appreciated

 

Thanks i did check that, and they are enabled..... its odd that it only does it sometimes?

There is definately correlation between the safari and hard drive problems...... If you were troubleshooting these, that would be undisputable lol.