found the solution:
the problem was in fstab because i was stressed i did look for it in the wrong place with a kubuntu installation usb drive i was able to boot in the try-it-out version and then i could mount the drive then wen to the "root" drive (which confusingly contais a "root" folder which is locked, but turnes out i don't need that one anyway) in that folder is the "etc" folder in which is a "fstab.txt" I checked with partition manager what "UUID-number" the boot-drive has, i also remembered from the error message and from mounting the external-drive (which i mentioned in my question) the UUID-number of said drive i uncommented ("#" in front of everything) whatever was added in the last line because there was the number of the external-drive and further up was the uuid-number of the boot-drive restarted and everything was fine
=)
