Progress during large file copy (Copy-Item & Write-Progress?)

It seems like a much better solution to just use BitsTransfer, it seems to come OOTB on most Windows machines with PowerShell 2.0 or greater.

Import-Module BitsTransfer
Start-BitsTransfer -Source $Source -Destination $Destination -Description "Backup" -DisplayName "Backup"

I haven't heard about progress with Copy-Item. If you don't want to use any external tool, you can experiment with streams. The size of buffer varies, you may try different values (from 2kb to 64kb).

function Copy-File {
    param( [string]$from, [string]$to)
    $ffile = [io.file]::OpenRead($from)
    $tofile = [io.file]::OpenWrite($to)
    Write-Progress -Activity "Copying file" -status "$from -> $to" -PercentComplete 0
    try {
        [byte[]]$buff = new-object byte[] 4096
        [long]$total = [int]$count = 0
        do {
            $count = $ffile.Read($buff, 0, $buff.Length)
            $tofile.Write($buff, 0, $count)
            $total += $count
            if ($total % 1mb -eq 0) {
                Write-Progress -Activity "Copying file" -status "$from -> $to" `
                   -PercentComplete ([long]($total * 100 / $ffile.Length))
            }
        } while ($count -gt 0)
    }
    finally {
        $ffile.Dispose()
        $tofile.Dispose()
        Write-Progress -Activity "Copying file" -Status "Ready" -Completed
    }
}