My external drive is not opening anymore, but it gets recognized. As soon as I open my Filesystem I can see the entry "SAMSUNG" above "Other Location". However if I click on it, nothing happens...
It used to work just fine. Did I mess something up with the access permissions?
Just a guess, but a very likely one. Your SAMSUNG is formatted using the Microsoft Windows ntfs file system. If the file system is not "clean", Ubuntu will not mount it in order to prevent further damage to the file system.
Connect the external drive to a Windows system. Use the Windows drive checking tools to check the drive. Then properly remove the drive from the Windows system (use the tray icon to remove the drive before physically plugging the drive out), or fully shut down Windows (fast start should be disabled). The "clean" file system will mount next time you use it under Linux.
If you do not have Windows, then consider reformatting the drive to a file system that is fully supported by Linux. Typically, you would format in ext4.