I have a directory named /media/joe, with joe:joe 750 permissions. I'm
reasonably sure that systemd originally created that, but if you have
an appropriate directory which was created with different permissions,
that may be a problem.
Thank you! Mine was mostafa:root 755, I don't know why it was `root`, I changed it back to `mostafa` and now it clearly fine!
I have the right permissions and speed:
(I can run dd without sudo)
$ dd if=/dev/zero of="/media/mostafa/Transcend/largefile" bs=1M count=1000 1000+0 records in 1000+0 records out 1048576000 bytes (1.0 GB, 1000 MiB) copied, 50.4967 s, 20.8 MB/s