It can also be reproduced from parted with:
$ sudo parted (parted) mklabel msdos (parted) mkpartfs primary ext2 0 20 ign (parted) <--- XFCE automounts the new partition here
It can also be reproduced from parted with:
$ sudo parted
(parted) mklabel msdos
(parted) mkpartfs primary ext2 0 20
ign
(parted) <--- XFCE automounts the new partition here