[solved] Data-Backup to USB-disk needs the same space at local file-system for "security-copy"

It would be nice, if the backup-engine could check the space at the local file-system before writing the security-copy. If there is not enough space on it, it could give an error-message and ask the user if it’s okay when the backup is written to the external USB-disk directly without copying to the local file-system before or the user could have the opportunity to disable the security-copy completely.
The solution above seems to be useful in the case, that there is no external disk to store the backup. As it does not avoid writing backup-data to local file-system, if there is not enough space, it is not a solution for my problem, isn’t it? Maybe I’m wrong.