Cannot backup in full

[freethedata]

When I try to do a full backup to either of two external hard drives, E: or G:, I get the error message "Cannot complete backup.  Your system is low on disk space".  The Organizer computes the backup size as 210 GB, yet File Explorer shows 1119 GB free on E: and 795 GB free on G:, and 676 GB free on C:.  I still can do incremental backups to older full backups on these drives.   What's the problem and solution here for full backups?

[freethedata]

And i did find that the calculated size of the backup and the actual size of the backup matched; the size this time was 220 GB.  It looks like there were a lot of errors in the previous full backup;  i found a lot of different video files with exactly the same bloated size (obviously extremely unlikely), so i probably could not have done a successful incremental restore.  Maybe there was some kind of gradual corruption in the full backup process (although i don't understandwhy uninstall & reinstall didn't fix it).

[freethedata]

Or after it accurately calculated the size of the backup file, it went into some irregular process where it expanded the size of every video file, and there was no room on the drive for these erroneously inflated files,   

[MichelBParis]

In your screen shot, you show the C: drive as not active scratch disk, so I didn't check it. Will try again with C: checked.

Yes, I did not want my C drive to be used as a scratch disk because it had to little free space.

What I am trying to ascertain is that the message about not enough system disk space is really linked to the C drive. You have not assigned C as a scratch disk, I assume you have ticked one or both of the other drives. That's good since it means there is little risk of a lack of disk space for the system and little risk of a lack of disk space on C since it is not assigned and used as a scratch disk. So my initial hypothesis does not verify.

Your other problem of disk space calculation would point to not enough disk space on the destination drive, or rather an incorrect calculation of backup size. That would mean an error in the catalog. I'll take the time to read again all your tests and come back tomorrow.