Split-Path with Root UNC Directory

By using the System.Uri class and querying its host property:

$uri = new-object System.Uri("\\server\folder")
$uri.host # add "\\" in front to get exactly what you asked

Note: For a UNC path, the root directory is the servername plus the share name part.

An example using regular expressions:

'\\server\share' -replace '(?<!\\)\\\w+'