Create a binary file of a given size full of random data - useful when we want to test things like disk io / network performance of copying files, ensuring there are no patterns allowing for unwanted optimizations to kick in

New-DummyFile.ps1

function New-DummyFile {
    [CmdletBinding(SupportsShouldProcess = $true, ConfirmImpact = 'Medium')]
    Param (
        # full path to file to be created
        [Parameter(Mandatory = $true, ValueFromPipeline = $true)]
        [string]$Path
        ,
        # the size of the file to be created (you can use shorthand terms like 1GB to populate this)
        [Parameter(Mandatory = $true)]
        [UInt64]$FileSizeBytes # use uint to avoid negatives; use int64 to ensure we can hold values in terrabytes
        ,
        # create the file without any prompts, regardless of ConfirmPreference
        [Parameter()]
        [switch]$Force
    )
    Begin {
        [Int32]$DefaultPageSize = 1MB
        [Boolean]$ClobberYesToAll = $Force.IsPresent
        [Boolean]$ClobberNoToAll = $false
    }
    Process {
        [Boolean]$fileExists = Test-Path -Path $Path -PathType Leaf
        if ((-not $fileExists) -or ($PSCmdlet.ShouldContinue("File already exists: $Path.  Overwrite?", 'Overwrite File?', [ref]$ClobberYesToAll, [ref]$ClobberNoToAll))) { # if the file exists then prompt the user on whether to delete it
            # create a new file
            $splat = $PSBoundParameters
            $splat.Remove('FileSizeBytes') | Out-Null
            $splat.Add('ItemType', 'File')
            if (!$splat.ContainsKey('Force')) {$splat.Add('Force', $true)} # to reach this point the user's confirmed overwrite above, so we can treat the rest as forced
            $newItem = New-Item @splat
            
            # write some random data to it. Check that the new item exists since what-if may have caused no output, in which case we don't want to risk anything here
            if ($null -ne $newItem) {
                try {
                    [System.IO.BinaryWriter]$writer = New-Object -TypeName 'System.IO.BinaryWriter' -ArgumentList ([System.IO.File]::Open(($newItem.FullName), [System.IO.FileMode]::Create))

                    [UInt64]$NoOfFullPages = [Math]::Floor($FileSizeBytes / $DefaultPageSize)
                    [Int32[]]$pageSizes = @(@($DefaultPageSize) * $NoOfFullPages) 
                    [Int32]$Remainder = $FileSizeBytes - ($NoOfFullPages * $DefaultPageSize) 
                    if ($Remainder -gt 0) {$pageSizes += @($Remainder)}
                    [System.Random]$rnd = New-Object -TypeName 'System.Random'
                    foreach ($page in $pageSizes) {
                        Write-Verbose "Writing page of $page bytes $($page.GetType()) to $($newItem.FullName)" 
                        [Byte[]]$data = New-Object -TypeName 'Byte[]' -ArgumentList $page
                        $rnd.NextBytes($data); 
                        $writer.Write($data)
                    }
                    $writer.Flush()
                } finally {
                    if ($null -ne $writer) {
                        $writer.Dispose()
                    }
                }
            }
        }
    }
}

New-DummyFile -Path 'c:\temp\one.dat' -FileSizeBytes ((1GB)+5) -Verbose