For network shares you should use UNC names:
"//server/share/dir/file.ext"
Source
If you use the IP or hostname it should work fine:
$isFolder = is_dir("\\\\NAS\\Main Disk");
var_dump($isFolder); //TRUE
$isFolder = is_dir("//NAS/Main Disk");
var_dump($isFolder); //TRUE
$isFolder = is_dir("N:/Main Disk");
var_dump($isFolder); //FALSE