Mounting ext4 partition after boot is tricky. If it is mounted manually in terminal, sudo must be used. So, the partition would have been mounted as by root and user would not have write permission. In that case, to have write permission as user, do the following.
# chown username /path/to/mounted/dir
To avoide that situation, the best way is mounting the partition in /etc/fstab. In mounting ext4 partition in /etc/fstab, the main thing is user and noauto options. The example;
# /dev/sda1
UUID=63edf43c-b195-49da-bb5c-9f14c4e01570 /home/msh/001HDD ext4 rw,user,noauto,relatime 0 2
then do the following;
$ mount /home/msh/001HDD
or put mount /home/msh/001HDD in any where autostart. For instance, if using sway wm, put it in ~/.config/sway/config as follow;
exec mount /home/msh/001HDD
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