Often when you write the partition table back out, you'll get a notification that the kernel couldn't properly figure out what you just did to the disk, and so it will continue to use the old partition table -- and that you'd better not try to do anything to your disk until you've rebooted...

That's a pain --obviously-- but often you can avoid this by entering

partprobe

when you get this message. It tells the system to re-probe the disk (as though it were in the process of booting up) and often --not always-- will get the kernel to clear its head and "get" your new partition configuration.

That can save an undesirable reboot...