mjhjmtu

I created an udev rule that launches a shell script that creates a directory /media/my-disk when some specific USB HD is connected. It has been working many months, but after a reboot (probably some packages were updated) it shows the following message (in syslog) everytime I plug the drive in:

mkdir: cannot create directory '/media/my-disk': Read-only file system

I have added the following lines to the script:

whoami
mount

And the result is:

root

usb-mount.sh: /dev/mapper/sda3_crypt on / type btrfs (ro,relatime,ssd,space_cache,subvolid=257,subvol=/@)

So it seems udev is seeing the root filesystem as readonly, but if I execute sudo mount manually it shows it mounted as read/write:

/dev/mapper/sda3_crypt on / type btrfs (rw,relatime,ssd,space_cache,subvolid=257,subvol=/@)

I've searched on the Internet and I've found the same exact problem but without a solution:
https://www.raspberrypi.org/forums/viewtopic.php?t=210243

Restarting the udev service solves the problem until the next reboot.

What can I check solve it permanently?

NOTE: I am running Ubuntu 18.04.1 LTS