PowerShell $ + $
Solution 1:
you are trying to add two PSCustomObject
items to each other ... and that object type does not have an addition method. [grin]
what you want to add is the value in the single property of each object. this ...
$FDS.Free + $UDS.Used
... will give you what you want.
however, i would use something like ...
$CDriveInfo = Get-PSDrive -Name 'c'
$TotalSize_CDrive = $CDriveInfo.Free + $CDriveInfo.Used
$TotalSize_CDrive
... OR use Get-Volume
OR a CIM query since those have the drive capacity directly available.