Issue with Powershell parsing in 4.0 - powershell

Question

I am having issues with the snippet below with Powershell 4.0 on 2012 R2:
$query = 'ASSOCIATORS OF {Win32_DiskDrive.DeviceID="' +
$disk.DeviceID.replace('\','\\') +
'"} WHERE AssocClass=Win32_DiskDriveToDiskPartition'
On my Windows 7 laptop running Powershell 2.0 this works fine both in the Powershell cli and in a .ps1 script. In Powershell 4.0 if I run it in the cli it works fine, but if I run it out of a .ps1 script I get:
You cannot call a method on a null-valued expression.
At C:\temp\perf.ps1:94 char:1
+ $part_query = 'ASSOCIATORS OF {Win32_DiskDrive.DeviceID="' + $disk.DeviceID.rep ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: ( [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : InvokeMethodOnNull
After some debug I think the issue is with .replace('\\','\\\').
I have tried .replace('\\\','\\\\\\\') but I get the same error.
Full code:
$diskdrives = get-wmiobject Win32_DiskDrive | sort Index
foreach ( $disk in $diskdrives ) {
write $disk
$scsi_details = 'SCSI ' + $disk.SCSIBus + ':' +
$disk.SCSILogicalUnit + ':' +
$disk.SCSIPort + ':' +
$disk.SCSITargetID
#$part_query = 'ASSOCIATORS OF {Win32_DiskDrive.DeviceID="' + $disk.DeviceID.replace('\','\\') + '"} WHERE AssocClass=Win32_DiskDriveToDiskPartition'
$part_query = 'ASSOCIATORS OF {Win32_DiskDrive.DeviceID="' + $disk.DeviceID.replace('\\','\\\') + '"} WHERE AssocClass=Win32_DiskDriveToDiskPartition'
$partitions = #( get-wmiobject -query $part_query |
sort StartingOffset )
foreach ($partition in $partitions) {
$vol_query = 'ASSOCIATORS OF {Win32_DiskPartition.DeviceID="' +
$partition.DeviceID +
'"} WHERE AssocClass=Win32_LogicalDiskToPartition'
$volumes = #(get-wmiobject -query $vol_query)
foreach ( $volume in $volumes) {
$allDiskInfo += $($volume.name + ' ' +
$volume.FileSystem + ' ' + $disk.Index + ' '
) | Out-String -stream
} # end foreach vol
} # end foreach part
} # end foreach disk

Answer 1

When I declared $disks I made it [string]. What I think happened is that Powershell 2.0 coerced it back to the system object but Powershell 4.0 must have stronger typing rules and was not coercing it from what I declared it as. Once I took [string] out of the declaration, it works on 4.0.
Thanks for the responses!!