Fix NotSupportedException: Resolve multiple drive letter assignments and mount delays

[hayashibob]

Describe the bug

When running Update-VMGpuPartitionDriver.ps1 on modern Windows 11 environments with automount enabled, mounting the VM's VHDX often assigns drive letters to multiple partitions simultaneously (such as the OS partition and the Recovery partition).

Because the original code uses ForEach-Object DriveLetter, $DriveLetter receives an array (e.g., "E", "F"). When appending :, PowerShell concatenates the array with spaces, resulting in an invalid path format like "E F:\windows\system32". This causes Copy-Item to crash completely with a NotSupportedException.
Additionally, there is a timing issue where the script might execute before Windows fully recognizes the file system, or the disk remains Offline depending on the SAN policy, causing the variable to be empty.

Proposed Solution

I rewrote the mount and drive letter acquisition block to be foolproof:

1. Added a 3-second Start-Sleep immediately after Mount-VHD to allow the file system to initialize.
2. Added a check to force the disk Online if it mounts as offline.
3. Used Sort-Object Size -Descending | Select-Object -First 1 to strictly pinpoint the primary OS partition, ignoring small recovery partitions.
4. If no drive letter is automatically assigned to the OS partition, it forces an assignment and retrieves it.
5. Ensured $DriveLetter is exactly one valid drive letter.

I have tested this patch extensively on my local Hyper-V environment (Win 11, RTX 4060 Ti) and it successfully transferred all driver files without any path formatting errors. I hope this helps others facing the same issue!