ls -la /mnt/hd500/dietpi_userdata/mysql
total 122968
drwxr-xr-x 7 mysql mysql 4096 Jun 22 21:09 .
drwxr-xr-x 9 root root 4096 Jun 22 16:25 ..
-rw-rw---- 1 proftpd mysql 16384 Jun 22 16:35 aria_log.00000001
-rw-rw---- 1 proftpd mysql 52 Jun 22 16:35 aria_log_control
-rw-r--r-- 1 root root 0 Jun 22 16:35 debian-10.3.flag
-rw-r--r-- 1 mysql mysql 0 Jun 22 21:14 demo.file
-rw-rw---- 1 mysql mysql 900 Jun 22 16:35 ib_buffer_pool
-rw-rw---- 1 proftpd mysql 50331648 Jun 22 18:54 ib_logfile0
-rw-rw---- 1 proftpd mysql 50331648 Jun 22 15:25 ib_logfile1
-rw-rw---- 1 proftpd mysql 12582912 Jun 22 16:35 ibdata1
-rw-rw---- 1 mysql mysql 12582912 Jun 22 18:55 ibtmp1
-rw-rw---- 1 proftpd mysql 0 Jun 22 15:26 multi-master.info
drwx------ 2 proftpd mysql 4096 Jun 22 15:26 mysql
-rw-rw---- 1 root mysql 16 Jun 22 15:26 mysql_upgrade_info
drwx------ 2 110 mysql 12288 Apr 2 17:11 nextcloud
drwx------ 2 proftpd mysql 4096 Jun 22 15:26 performance_schema
drwx------ 2 110 mysql 4096 Nov 28 2019 phpmyadmin
-rw-rw---- 1 mysql mysql 24576 Jun 22 18:55 tc.log
drwx------ 2 proftpd mysql 4096 Jun 22 15:27 wordpress
root@DietPi:~#
there you go. Files and folders are owned by user proftpd. There are as well folders with unknown user ID 110. Did you have done anything with FTP and/or copied data??
You can do following to change owner back to user mysql