This is the WMI query for DiskWritesPerSec:
SELECT AvgDiskSecPerTransfer, AvgDiskSecPerTransfer_Base, AvgDiskQueueLength,
DiskBytesPerSec, DiskReadBytesPerSec, DiskReadsPerSec, DiskTransfersPerSec,
DiskWriteBytesPerSec, DiskWritesPerSec, Frequency_PerfTime, Frequency_Sys100NS,
Name, PercentDiskTime, Timestamp_Sys100NS
FROM Win32_PerfRawData_PerfDisk_PhysicalDisk
WHERE Name <> "_Total"