Renaming Folders ascending - powershell

Question

I want to automate my backups and Keep always some old versions. The idea was to use Windows Backup on a share and use a PowerShell script to start it.
I'm almost done, but I stuck at the renaming.
Share: \\Server\SysBackup\WindowsImageBackup
in that share there are Folders for all PC's I have. So for example I want to Keep the three last backups, it should to the following:
Current Backup: PC1
Old Backups: PC1_1, PC1_2
Now I want to rename them to one higher number
PC1_2 → PC1_3
PC1_1 → PC1_2
PC1 → PC1_1
So the backup now can use the PC1 Folder for the newest backup.
That's what I tried so far:
$BackupFolder = Get-ChildItem -Directory ($Target + "\WindowsImageBackup") |
Where-Object -Property Name -Like $env:computername* |
Select-Object -Property Name
$CurrentBackups = $BackupFolder.Count
if ($CurrentBackups -ge 1) {
Push-Location ($Target + "\WindowsImageBackup")
$i = 0
$xCurrentArray = $CurrentBackups - 1
$NewSubVersion = $CurrentBackups
while ($i -le $CurrentBackups) {
$NewName = $BackupFolder[$xCurrentArray].Name.TrimEnd("_")
Rename-Item $BackupFolder[$xCurrentArray] -NewName
}
Pop-Location
Clear-Variable $i
}
The files are not renamed, and I'm getting the following error:
You cannot call a method on a null-valued expression.
Rename-Item : Missing an argument for parameter 'NewName'. Specify a parameter of type 'System.String' and try again.
Where is my mistake?

Answer 1

I found the error
if ($CurrentBackups -ge 1)
{
Push-Location ($Target + "\WindowsImageBackup\")
$i= 0
$xCurrentArray = $CurrentBackups - 1
$NewSubVersion = $CurrentBackups
while ($i -lt $CurrentBackups)
{
if ($BackupFolder[$xCurrentArray].Contains("_"))
{
$Index = $BackupFolder[$xCurrentArray].IndexOf("_")
$NewName = $BackupFolder[$xCurrentArray].Substring(0, $Index)
Rename-Item $BackupFolder[$xCurrentArray] -NewName ($NewName + "_" + $NewSubVersion)
Clear-Variable Index
$NewSubVersion--
}
else
{
$NewName = $BackupFolder[$xCurrentArray]
Rename-Item $BackupFolder[$xCurrentArray] -NewName ($NewName + "_1")
}
$i++
$xCurrentArray--
}
Pop-Location
Clear-Variable i
}