Maximus83, it looks like you solved your issue, without changing out the SD Card in your Clara HD. Just because I 'could' I updated in my Clara HD from the existing 8gb to a 64gb. I followed the instructions exactly in this thread (note, I use Windows, if you use something else you'll probably need different instructions):
https://www.mobileread.com/forums/sh...d.php?t=261362
I used
Samsung UHS-I Evo Plus, 64GB; because of an article I can't find anymore where someone compared a lot of different sd cards, and for the small file writes, the Evo Plus was the best in his opinion.
I did have one issue--which someone else had as well. After transferring and extending, and adding it into the Clara HD; when connecting to the computer, the computer would prompt me that the drive had a problem and needed to be 'fixed'. I did a factory reset on the Clara HD (yes, I lost everything...); and the prompt on the PC when connecting to Clara HD no longer occurs.