partitionmanager as root but no permission to create a mount folder

[trotosa]

Excuse me for my english. It's not my native langage.

With kubuntu 18.04 beta1.

I am using partitionmanager with uid 0, gid 0, (verified with ksysguard).

When I try to change the mounting point, I can only create a new folder with my user uid.
So The mounting must be in $HOME or /media/$USER.

I don't understand why root couldn't define a mounting point for exemple /mydisk.

Then, as the process is with root uid, the file /etc/fstab could be changed but not with the wanted mount point.

I made an other test by changing /usr/share/applications/org.kde.partitionmanager.desktop and add :
X-KDE-SubstituteUID=true
X-KDE-Username=root

But no change, I am still with my user id when I try to create the mounting point.