[Kazun]

Для начала не плохо бы озвучить версию PowerShell: The ReadOnly parameter was introduced in Windows PowerShell 3.0.

PS > Get-help Get-ChildItem -Path C:\ -Parameter ReadOnly

-ReadOnly [<SwitchParameter>]
Gets only read-only files and directories (folders).

To get only read-only items, use the ReadOnly parameter, its "ar" alias, or the ReadOnly value of the Attributes pa
rameter. To exclude read-only items, use the Attributes parameter.

Все мои примеры v3 и выше.