Disk transfer time measures the time it takes in milliseconds to transfer data between disk and memory. The metric includes disk reads and disk writes.
If disk transfers are taking consistently longer than 50 milliseconds a disk bottleneck may be developing. The Disks - Transfer Time alarm is raised when the transfer time exceeds 50 milliseconds.
When the alarm is raised
Refer to the Windows Server | Disks Drilldown| Physical Disk Activity page | Transfer Time chart.