Thanks for pointing me into this direction.

When I use an online calculator to convert Gibibytes to Gigabytes, I get this result: 634.8 GiB are 682 GB. So, exactly the size that the Gnome-Disk-Utility shows.

The App Stacer tells me 624.2 GiB. When I convert this into Gigabytes, I get 670.3 GB. This size is also shown in the Pop_OS file manager.

So, in conclusion:

• fdisk command and Gnome disk utility are showing 634.8 GiB / 682 GB.
• Stacer and file manager are showing 624.2 GiB / 670.3 GB.

One step closer… Now I just need to figure out why two apps are showing too less storage space compared to the other ones.

Around the 2000s I hosted a Shoutcast server that played a playlist of about 30 punk rock MP3’s on continuous loop. As far as I can remeber, it was running on a Win2000 machine. Yeah - Pirate Radio 😆