I agree with tim.cloud that you should have a backup drive attached to your database system.
I normally set up the following when creating new SQL server
C: for OS ~ 60GB (nothing else on this than primary and shared SQL stuff)
D: allways CD or DVD drive
E: Datafiles (ask or check initial size, annual growth and usage before deciding size and RAID)
F: Logfiles (I choose 1,5 times the size of calculated Datafiles, at least RAID 5)
G: backup (I choose at least 2 times the size of calculated Datafiles)
Considerations has to be made of instances, Analyzing Services and such, depending on how many drives one should have.
There is also on heavily loaded system sometimes a need for a TEMPDB drive.