Thanks to @devnull in this thread (Cron job MySQL disk writes - #12 by devnull) I was able ro resolve it.
I confirmed that the cron job is triggering the access to the drive by running it manually
sudo -u www-data php /path/to/nextcloud/cron.php
Then I created a log file with strace during running
sudo -u www-data strace -ytff -o /tmp/log.txt php /path/to/nextcloud/cron.php
I did this a few times and always checked with grep which files are accessed on my external drives.
Based in the log file I was able to identify that this was related to an issue with the index of the elasticsearch (fulltextsearch) index which was broken and cron always wanted to recreate it.
I deinstalled elasticsearch for now and now everything is normal again. Will try to install it later again.
Thanks for your support.