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Sorry to hear you’re facing problems 
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Nextcloud version: 22.1.1
Operating system and version: Ubuntu 21.04
Apache or nginx version: Apache 2.4.45
PHP version : 8.0.10
The issue you are facing:
I have a RaspberryPi using SD card for boot. It is small and I want to upgrade it to a larger usb stick. I use docker to run nextcloud. The /var/www/html/data volume is linked to an external HDD drive. I want the drive to remain, I am only swaping the SD card.
I cannot figure out how to create new nextcloud container on the USB such that I can access the data on the drive. If I try to use creadentials from the SD card I get an error that such user already exists. I presume there is some folder that should be copied from one container to another but I can’t figure out which one.
I am using MariaDB and I have tried making a backup using SD card and restoring using the USB stick but it didn’t work.
I still have the SD card working. If I use it to boot everything works.
The output of your config.php file in /path/to/nextcloud
$CONFIG = array (
'htaccess.RewriteBase' => '/',
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'overwriteprotocol' => 'https',
'instanceid' => '[XxX SENSIBLE CONTENT REMOVED BY MOD/JK XxX]',
'passwordsalt' => '[XxX SENSIBLE CONTENT REMOVED BY MOD/JK XxX]',
'secret' => '[XxX SENSIBLE CONTENT REMOVED BY MOD/JK XxX]',
'trusted_domains' =>
array (
0 => 'nextcloud.mydomain.com',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'mysql',
'version' => '22.1.1.2',
'overwrite.cli.url' => 'https://nextcloud.mydomain.com',
'dbname' => 'nextcloud',
'dbhost' => 'mariadb:3306',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud',
'dbpassword' => 'password',
'installed' => true,
);