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The Most Active Disk gauge in the Disk Activity panel shows the disk with the highest read/write rates.
The Percentage Busy alarm is triggered when the read/write rate of the most active disk exceeds a specified threshold.
A disk that is more than 20% busy (according to Sun Performance and Tuning by A. Cockroft) should be investigated, and possibly have data on it split up and moved to different disks.
The type of alarm that is activated is determined by the percentage of read/write activity experienced by the disk.