NT/2000 & RAID 5 - With old disks the figure to watch for concerning % disk time was somewhere between 30 - 40%. Any ideas what this should be for RAID 5 disks?

30-40% utilization is right in that it's a good performance figure. As for RAID 5, well, it's just a collection of disks that should be dealt with individually in the same way, so 30-40% is good. Unfortunately the way in which the array is reported back via performance monitors varies and one can never be certain what is being seen. However, proprietary monitors may allow individual disks to be examined. In any event, the rule of thumb for a disk applies whatever configuration it exists in.

The differences obviously come into play when considering the type of work being performed. With RAID 5 for example, higher utilization will be seen across the array when writing data because of the need for the additional operations required to write the parity stripe. So if different operations take place higher utilization figures may be acceptable.

What may be more important is the actual response time of the device (seek+rotation+data transfer+queue) if it can be accurately reported. This in tandem with knowledge of the type of operation taking place can be a more accurate picture of how an array is performing.