Disk Separation DB installation and data files
Why do we separate things onto different disks?
Performance and resilience.  

From the Disaster Recovery perspective, if you lose any one of your disks, how much damage does it do? 

Lose the operating system disk?  OK, that's a biggy. :-(

Lose the backup disk?  Non-event; you still have a running database instance.  

Lose the data disk?  Ouch!  But that's OK; you've got your backups ... 
You just need to restore them into a running SQLServer instance ...  

Ah.  

But you had that on the same [data] disk that just failed, taking the data with it!  

Keep the installation and data separate. 

Whilst it's only semantics, I'd suggest keeping a closer "association" between the Data and Logs disks; they make up all the stuff you really care about. 

Move the backups off to another server altogether.  (Whilst it's unlikely these days) If all these disks are local to the machine and the machine's motherboard were to fail, it wouldn't matter how many disks you had; they'd all be gone!