Same VM has multiple 30GB disks attached? Try to run the PowerCLI commands can fetch the RDM list first. In the output, Name field gives hard disk number through which we can try to identify the OS disk.
Get-VM | Get-HardDisk -DiskType "RawPhysical","RawVirtual" | Select Parent,Name,DiskType,ScsiCanonicalName,DeviceName | Out-File –FilePath RDMsVMs.txt