No write access to backup folder for BIT instance on another computer

Basically, I have set up BIT to back up my home folder /home/user/. I have chosen Local without encryption. BIT is set up to backup to an external HDD, and it seeminly works perfectly. But I get trouble when I reboot and start another Linux distro (Ubuntu 16). I point BIT to the config file in the backup folder on the ext. HDD, and the config is recognised. However when I choose OK BIT claims that it does not have write access to the backup folder. Also, when I manually browse to the backup folder the contents of the home folder I backuped up are locked.

Am I doing something wrong?