You have a Full backup of 1.5TB on a Linux system that runs once a week, the rest of the week, Differentials run.
The differential backup following the full and all others are taking as long or longer than the full backup. Also, the amount of data getting backed up is equivalent to the Full backup.
There are several reasons why this can occur:
1. The Incremental/Differential option in the "Backup Options" set was not selected.
2. You are backing up remote mounts. When these are backed up, NV does not go into these to check for the changed files, it merely checks the mount point.
3. Your OS is changing directories to mount points daily, forcing the folders to be completely backed up.
4. Aggressive AV scans could affect the large volumes and thus force the whole volume to update the change time on the files, forcing all files to be backed up.
The most common scenario out of the above is #2. Always make sure that when mount points are involved in a backup that " Backup Through Remote Mounts " is selected. If you have any reservations about this, selecting anway will have no adverse effect. This will force the plugin to scan through the mount point to see the changed files and not just the root volume.
As for the other scenarios, check for these things as well. If you notice in your Inc/Diff that it is always very large when it should be, then find out whats being backed up by creating a " Backup Log" and seeing if there are things getting backed up daily that are not actually changing.
Also, aggressive security software can update entire volumes and their subfolders and files. Please check your AV software to verify if it is modifying files enough to force a change. If so, make sure you throttle down some settings on your AV if possible.