I have one external hard disk and I bought another one to make a backup of the first one.
I thought that the dd command would do the trick to make a copy of the first external harddisk. That didn't turn out that well. Now I cannot access the second harddisk.
This is the output of lsblk:
The output of sudo fdisk -l /dev/sdc gives:
So, the disk is recognized but I cannot access it. What to do?

We don't know what you did ("a dd command" isn't an answer), what the messages were, and what you mean by "I cannot access it".

You appear to have been here long enough to know you need to provide all the necessary info to get a reasonable answer.

Update: some errormessages
Now I have an errormessage. I found this page to help me out:
https://help.ubuntu.com/community/In...ition_The_Disk

If I understood it correctly, I must make a mount point and take it from there.

Thank you for your answer, syg00. I consider myself still a newbie. I can google to find some answers and can run some commands in the terminal, but I am not a pro and therefore I don't always know what others need to know to answer my question.

I will try to solve it at some later time and will post my findings in this post.

You need to mount the partition - actually the filesystem on the partition - not the base device. And yes, you need a mount-point to attach it to - /mnt should exist. Not generally recommended as a long-term usage, but ok to ensure things are ok as a test.If you get any messages, post them here.

The command sudo mount /dev/sdc1 /mnt gives:
The command lsblk -f gives:

I found this:
https://www.howtogeek.com/235655/how...rive-on-linux/

I will try it later. I really have to go. Thanks again, syg00

So you used dd to copy a disk to another. Was the original exactly the same size as the destination? If not then dd can be an enemy.

If you still have the original then I would use something like fdisk, gparted, etc to delete all partitioning and write a new gpt partition table to the new disk. Then partition it properly, along with formatting the new partition(s).
Then I would create 2 directories under /mnt such as /mnt/old-disk and /mnt/new-disk and mount the partitions from the old disk and new disk as named.
Finally I would use something such as rsync to copy the data from one to the other. Something such as "rsync -av /mnt/old-disk/ /mnt/new-disk/" would work very well.

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I used gparted to partition the external hard disk and was able to copy my files to the new disk.
Thank you, computersavvy and syg00 for your answers.