I may already have an idea. By running this command:
ls -l /mnt/mongoDB-One/DB_Data_1st/* | grep -v mongodb
I found a few files with root onwnership. Maybe because I once ran mongod as root for test.
Anyway, I then reran:
sudo chown -R mongodb:mongodb /mnt/mongoDB-One/DB_Data_1st
and that seems to solve the problem. It means you were right to suspect some permission issue.
But there is something else I noticed. If I have these two lines in the config file:
processManagement:
fork: true
then the server starts and then vanishes after a short while. On the other hand if I delete them, all seems to be fine. Do you know if this is normal?