There are 2 reasons that are most likely to be the cause of a nonesense bundletag and size error.
A nonsense bundletag and size error basically means that the indexes of the backup file are not what we expect. This can be caused by:
- bad media
- bad writes to media
- packet loss
- delayed write failures
- other programs trying to 'scan' the files as they are being written causing checksum issues
- or other issues that may affect the write process of the backup.
Depending on where you save your data, take precautions and run maintenance on these devices/media.
You can try to use disk defragment on the drive or windows error checking to correct this, but it may not be enough. Unfortunately some drives reliability can suddenly drop, hopefully yours can be corrected with those simple steps.
Additionally, a nonsense bundletag error can be caused by the archive flag not being present in one or more files being backed up during and incremental or differential backup. If this is happening to you, try performing a full backup instead of an incremental or differential.