Synology NAS - Insufficient capacity for update. The system partition requires at least xxx MB

SSH to the Synology NAS server.

ssh admin@[ip address]

Enter password.

Type sudo su

Enter password.

Now search for the large files by doing the following:

1. type: cd /
2. type: du -hs [a-u]* va*

The last command should list the size of each first level directory, excluding the /volume<something> where your data is stored

Find the biggest directory, then go down one level with "cd <directory_name>"
and try a "du -hs *" to repeat the process for all subdirs.