Copy everything except files on the list - powershell

Question

I am trying to copy all files recursively from a:\ to b:\, except those whose metadata is present in a:\list.txt. The list.txt pattern is LastWriteTimeYYYY-MM-DD HH:MM:SS,size,.fileextension, for example:
2001-01-31 23:59:59,12345,.doc
2001-01-31 23:59:59,12345,.txt
2001-01-31 23:59:00,456,.csv
...so any and all files, anywhere in the a:\ dir tree, matching these metadata should not be copied.
I seem to be having trouble with the Where-Object in order to exclude the items on the list.txt, but copy everything else:
$Source = "C:\a"
$Target = "C:\b"
$List = Import-Csv list.txt -Header LastWriteTime,Size,Name
$Hash = #{}
ForEach ($Row in $List){
$Key = ("{0},{1},.{2}" -F $Row.LastWriteTime,$Row.Size,$Row.Name.Split('.')[-1].ToLower())
IF (!($Hash[$Key])) {$Hash.Add($Key,$Row.Name)}
}
$Hash | Format-Table -Auto
Get-Childitem -Path $Source -Recurse -File | Where-Object {$Hash -eq $Hash[$Key]}| ForEach-Object {$Key = ("{0},{1},{2}" -F ($_.LastWriteTime).ToString('yyyy-MM-dd HH:mm:ss'),$_.Length,$_.Extension.ToLower())
#$Key
If ($Hash[$Key]){
$Destination = $_.FullName -Replace "^$([RegEx]::Escape($Source))","$Target"
If (!(Test-Path (Split-Path $Destination))){MD (Split-Path $Destination)|Out-Null}
$_ | Copy-Item -Destination $Destination
}
}

Answer 1

I propose you a simplification of your code :
$Source = "C:\a\"
$Target = "C:\b\"
New-Item -ItemType Directory $Target -Force | Out-Null
$List = Import-Csv list.txt -Header LastWriteTime,Length,Extension
Get-Childitem $Source -Recurse -File | %{
$File=$_
$exist=$List | where {$_.LastWriteTime -eq $File.LastWriteTime.ToString('yyyy-MM-dd HH:mm:ss') -and $_.Length -eq $File.Length -and $_.Extension -eq $File.Extension} | select -first 1
if ($exist -ne $null) {continue}
New-Item -ItemType Directory $File.DirectoryName.Replace($Source, $Target) -Force | Out-Null
Copy-Item $File.FullName $File.FullName.Replace($Source, $Target) -Force
}