Check Whether a USB Drive Really Has the Capacity It Reports

The reported size comes from the controller, not from the memory. That is why a counterfeit stick shows the full size in Explorer and fails later.

Get-Disk | Where-Object BusType -eq 'USB' | Select-Object Number,FriendlyName,Size,PartitionStyle,SerialNumber
Get-Volume | Where-Object DriveType -eq 'Removable' | Select-Object DriveLetter,FileSystem,Size,SizeRemaining
Get-CimInstance Win32_DiskDrive | Where-Object InterfaceType -eq 'USB' | Select-Object Model,Size,SerialNumber

The test overwrites the drive. Copy the existing content off first, and verify the copy before you start.

$src = 'E:\'
$dst = "$env:USERPROFILE\usb-backup"
New-Item -ItemType Directory $dst -Force | Out-Null
robocopy $src $dst /E /R:1 /W:1 /NFL /NDL
(Get-ChildItem $dst -Recurse | Measure-Object Length -Sum).Sum

**fsutil file createnew** writes zeros quickly and uses the full reported capacity. A write that fails near the end already proves the drive cannot hold what it claims.

$free = (Get-Volume -DriveLetter E).SizeRemaining
$freeGB = [math]::Round($free/1GB,2)
fsutil file createnew 'E:\fill.bin' $free
Get-Item 'E:\fill.bin' | Select-Object Name,Length
Get-Volume -DriveLetter E | Select-Object SizeRemaining

The data is hashed on the way in and on the way out. A mismatch marks the point where the controller started lying, and it only appears after the real capacity.

$in = (Get-FileHash 'E:\fill.bin' -Algorithm MD5).Hash
$in
Copy-Item 'E:\fill.bin' 'C:\temp\fill-check.bin' -Force
$out = (Get-FileHash 'C:\temp\fill-check.bin' -Algorithm MD5).Hash
$out
if ($in -eq $out) { 'match' } else { 'MISMATCH - drive is fake or failing' }

A fake controller often handles one large file better than many small ones. A thousand small files across the capacity is the more realistic test.

1..1000 | ForEach-Object { [System.IO.File]::WriteAllText("E:\t$_.txt", ("x" * 1024)) }
Get-ChildItem E:\ -Filter 't*.txt' | Measure-Object | Select-Object Count
Get-ChildItem E:\ -Filter 't*.txt' | Measure-Object Length -Sum | Select-Object Sum

A drive that fails the read-back should not hold anything you care about. Use it for a scratch copy, label it, and never for a backup.

Get-ChildItem E:\ -Filter 'fill.bin' | Remove-Item -Force
Get-ChildItem E:\ -Filter 't*.txt' | Remove-Item -Force
Get-Volume -DriveLetter E | Select-Object SizeRemaining,Size
Get-Disk | Where-Object BusType -eq 'USB' | Select-Object FriendlyName,HealthStatus