Q: File Vault

While trying to Update El Capitan from the App Store, it says I need to restart my computer.  When I click to restart, the File Vault comes up to back up my computer.  It always gets stuck and never restarts.  I wind up having to unplug it (and know that's a no no, but it's the only way to get it to restart).  Any suggestions on how I can fix this?

I have a 2008 iMac desktop.  4g of memory.  225G of available space. 

When I do this same process on my 2009 MacBook Pro, it works perfectly.  I'm not very technical in this department.  How do I fix File Vault?

Thank You