VSS on an external drive -> full?

I need to backup a windows PC with an USB (Thunderbolt) external disk with
6TB of data (disk Z:).


Sometime VSS complain:

 pug-ibr-eeg2ril-fd JobId 27870: Generate VSS snapshots. Driver="Win64 VSS"
 pug-ibr-eeg2ril-fd JobId 27870:     Snapshot mount point: D:\
 pug-ibr-eeg2ril-fd JobId 27870:     Snapshot mount point: C:\
 pug-ibr-eeg2ril-fd JobId 27870: Fatal error: VSS CreateSnapshots failed. ERR=Operazione completata.
 pug-ibr-eeg2ril-fd JobId 27870: Fatal error: vss.cpp:507 Backup is not Initialized
 pug-ibr-eeg2ril-fd JobId 27870: Error: VSS API failure calling "BackupComplete". ERR=Object is not initialized; called during restore or not called in correct sequence.
 pug-ibr-eeg2ril-fd JobId 27870: Warning: VSS Writer "MSSearch Service Writer" has invalid state. ERR=The writer vetoed the shadow copy creation process during the backup preparation state.
 pug-ibr-eeg2ril-fd JobId 27870: Warning: VSS Writer "Shadow Copy Optimization Writer" has invalid state. ERR=The writer vetoed the shadow copy creation process during the backup preparation state.

if i retry the job (and incremental one) sooner or later VSS work and backup
start effectively. Still listed as 'Incremental', but currently backing up
2TB of data and more to come.

 27870  Back Incr      2,242    1.929 T PUG-IBR-EEG2Ril   is running

It is not possible that someone modify 2TB of data.


Backup of external media need some specific setup?


Thanks.

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