Clean-LightroomDeletedMemoryCardFiles.ps1

# TODO: replace this hard-coded path with the path on disk where dngs are stored for current trip
$tripLocalPath = 'C:\Users\Ethan\Pictures\Curacao 2013'
$tripLocalFilter = '*.dng'

$memoryCardPath = 'd:\\'
$memoryCardFilter = '*.orf'

$names = dir $tripLocalPath -Recurse -Include $tripLocalFilter |
  Select -ExpandProperty BaseName |
  Sort-Object

$memoryCardFiles = Get-ChildItem $memoryCardPath -Filter $memoryCardFilter -Recurse

$keepers = $memoryCardFiles | ? { $names -icontains $_.BaseName }
$toDelete = $memoryCardFiles | ? { $names -inotcontains $_.BaseName }

if ($names.Length -ne $keepers.Length)
{
  throw "Memory card keeper count does not match local keeper count! Disk: [$($names.Length)] / Card: [$($keepers.Length)]"
}

if ($memoryCardFiles.Length -ne ($names.Length + $toDelete.Length))
{
  throw "Memory card file count [$($memoryCardFiles.Length)] does not match on disk keepers [$($names.Length)] + files to delete [$($toDelete.Length)] count!"
}

switch (Read-Host -Prompt "Really delete [$($toDelete.Length)] files from $memoryCardPath? [y/n]")
{
  'y'
  {
    $toDelete |
      % {
        Write-Host "Deleting $($_.FullName)!"
        $_ | Remove-Item
      }
  }
  default
  {
    Write-Host 'Not deleting files'
  }
}

Has some simple checks to ensure you don't delete stuff when things don't match up... Add a -WhatIf on line 34 to see the list of files to be deleted.