Can't you use a sensor which monitors transfers/sec ? This is doable via custom wmi sensor.
select DiskTransfersPerSec from Win32_PerfRawData_PerfDisk_PhysicalDisk where name='1 D:'
In order to do this, create a text file with the file extension .wql
Place the query above in there like so:
select DiskTransfersPerSec from Win32_PerfRawData_PerfDisk_PhysicalDisk where name='<#PH1>'
Place that file in your prtg install dir. Something like: C:\Program Files (x86)\PRTG Network Monitor\Custom Sensors\WMI WQL scripts
Then open up prtg and create the sensor. Fill in the PH1 value in prtg and you should be good to go. A great little util I use for fiddling with WMI is wmiexplorer which is available via http://www.ks-soft.net/