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I believe your question is already answered in our documentatio.

The root cause of your issue is that you must not use the Authentication - Step 1 button on each one of your sites:

When you want to connect more sites to Google Drive please copy the Refresh Token from another site linked to the same Google Drive account to your site's Refresh Token field.

Moreover, you are given the following warning regarding concurrent backups:

Since all of your sites are using the same Refresh Token to connect to Google Drive you must NOT run backups on multiple sites simultaneously. That would cause all backups to fail since one active instance of Akeeba Backup would be invalidating the Access Token generated by the other active instance of Akeeba Backup also trying to upload to Google Drive. This is an architectural limitation of Google Drive.

I know this sucks for having multiple sites upload to Google Drive. Unfortunately, it's how Google decided to architecture their Google Drive implementation: you get one connection per Google Drive user account and developer. Other storage services such as Dropbox and OneDrive allow multiple connections which makes far more sense for using them with Akeeba Backup.

FTP has different restrictions. Since FTP transfers through PHP do not support appending to files you need to use a small part size for split archives (around 10MB) to prevent PHP timeouts, as explained in our documentation.

A better approach is using a different remote storage service which supports chunked uploads and connection of multiple sites on the same account such as Dropbox, OneDrive, Amazon S3, BackBlaze B2, RackSpace CloudFiles or Microsoft Azure BLOB Storage to just name a few off the top of my head.