The partition table on device PS3DA was unreadable.
It also warned me that it would destroy all the data on the drive.
I didn’t like the idea of installing and updating all those games again, so I googled for this warning. I found out that YDL doesn’t have access to the entire disk, but only to the part it is allowed to by the PlayStation Firmware. So the only data it could destroy, was the data on the partition which I had created in the PlayStation 3 menu. This means it doesn’t destroy all data on the disk, but only on the empty partition, where no data is stored.
After the installation and booting into the GameOS, it seemed all my savegames and gamedata was still available. So no worries when you come across the warning I described above, your data will not be lost.
You still might want to backup your savegames before installing YDL, you never know what could happen…