The description in the Performance Analysis help is as follows: "The percentage of time the busiest disk spent serving system-wide I/O requests. This metric serves as a measure for the system I/O load. High values may indicate device bottleneck." Is there a way to determine which disk has the high utilization in PA?
The best way to determine which disk is most utilized for your system would be to open perfmon and add the following counters LogicalDisk for all disks where SQL Server is running:
% Disk Time
This will tell you how much the disk is working
Current Disk Queue Length
This will tell you whether there are jobs waiting in queue for disk time
This will tell you how many reads per second this disk is performing
This will tell you how many writes per second this disk is performing
Once you've determined which disk is busiest, you can focus your search to the SQL Server applications you have running. Is your workload processing the issue or is your disk not keeping up with the volume? You can see whether a high amount of activity is too much for a disk by looking at SQL Server counters for the instance you're monitoring:
I/O log writes/sec
Is there a logging issue?
I/O batch writes/sec
Is there an issue with standard SQL processing?
I/O single-page writes
Is your data being written in a sub-optimal fashion?
If this value is high, you need to tune SQL Server.