If we have a server with  hard disks,and If we made  hards RAID 1+0 (c:)
install SQL on C:
It all depends on your I/O requirement. Best practice is to separate your data files and log files due to sequential vs random I/O, however if you are going to have multiple log files on the same drive then your sequential writes become more random. It all comes down to speed, size and protection that you are going for. I would personally go with a RAID 1 for C: and then RAID 6 for the secondary drive. SQL on C and Data and Logs on D. Again though, it depends on the work load, size of the data, etc.
answered Nov 15 '12 at 10:43 AM