When performing a migration, ShareGate Migrate displays the following error:

There is not enough free space on the SharePoint server's disk. This is usually due to the C: drive of the web front end server being full.

This error occurs because the SharePoint server does not have enough free space to write information on the disk. This is usually not related to the content database being full, but rather to the temporary location that SharePoint uses while it processes requests. For example, when uploading a file, SharePoint first stores it in a temporary folder before it saves it in the content database. If it cannot write to the temporary folder, it will give this error.

Find the server that does not have enough free space on its disk (usually the C: drive of the Web Front-End server, but it can be any server in the SharePoint farm) and either add more space to the disk or free some by deleting files that are no longer useful.