This indicates that this user is low on disk space. The number shown is how much space is still available. Please consider clearing out more space on this users computer before continuing to record.
Please see How is Available Disk Space Calculated
Hosts can archive old episode recordings from the dashboard to free up more space for future recordings.
If this issue is happening to a guest or co-host who has done many Zencastr recordings in the past, they can clear off any old backups that might be taking up space by following the instructions here: How do I delete past Zencastr recordings?