If you tried using WHM's Transfer Tool on very large sites, you've probably got a timeout error.
To fix this, we'll need to do this through SSH (via root) what the Transfer Tools wasn't able to do itself.
The Transfer Tool will create a /scripts directory--with all required scripts--at the base of your server, which we will use to create WHM/cPanel compatible backups.
The name of the script will look like the following: /scripts/pkgacct.YOUR_CPANEL_SERVER_NAME
Type the following to get the exact name:
ls -la /scripts/
Next, go into your /home directory to get a listing of the account names.