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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): replace me
Operating system and version (eg, Ubuntu 20.04): replace me
Apache or nginx version (eg, Apache 2.4.25): replace me
PHP version (eg, 7.4): replace me
The issue you are facing:
I have my nextcloud data on /dev/sbd2 (External USB Drive), but the disk usage grows at /dev/sda5 whenever I add files to nextcloud.
Before uploading a 2 gb file:
df -h /dev/sda5 /dev/sdb2
Filesystem Size Used Avail Use% Mounted on
/dev/sda5 49G 36G 11G 79% /
/dev/sdb2 7.3T 524G 6.8T 8% /media/go4nat/Seagate BPD 6TB
after uploading:
df -h /dev/sda5 /dev/sdb2
Filesystem Size Used Avail Use% Mounted on
/dev/sda5 49G 38G 8.9G 81% /
/dev/sdb2 7.3T 524G 6.8T 8% /media/go4nat/Seagate BPD 6TB
I have the config files as
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
'datadirectory' => '/media/usb/nextcloud-data',
'default_language' => 'en',
'default_locale' => 'en_US',
'default_phone_region' => 'US',
'dbtype' => 'mysql',
The output of your Apache/nginx/system log in /var/log/____:
PASTE HERE