The "Open File Backup" (VSS) backup job option (which is a default enabled backup job option, keep in mind) in NovaBACKUP client, if that option is enabled, to allow for open file backup to function in general, which uses VSS , for backup jobs (of any type and selection criteria), will not be able to function on a volume that is larger than 64 TB in size (according to Microsoft directly). If that is the case that the volume size is larger than 64 TB in size that the file backup job is attempting to backup (using either the legacy or the modern backup engine in the NovaBACKUP client) then it won't be able to take the VSS snapshot with that backup job option enabled, and any open files that are in the backup job selection will have to be skipped and not processed. This will also affect if your file backup job contains any plugins in it, including "System State", and "Hyper-V" and "Microsoft SQL", if for instance the MS SQL database(s) and/or Hyper-V VM(s), besides any files that may be in the backup job selection, are also located on a volume that is 64 TB or larger, for those customers that do not backup purely files in their file backup job in the NovaBACKUP client, but they also backup at least one plugin in the same backup job. To workaround this you could, starting in the 21.2.x client versions only (as prior client versions did not allow disabling that option, for the modern backup engine targeted backup jobs only), choose to disable the "Open File Backup" backup job option, by going to the "Backup" tab and then click on the "Settings" button at the bottom of that dialog, then in the "Backup" tab uncheck the "Open File Backup" backup job option there, and then save that backup job with that change, which will make it so that a VSS snapshot will not be attempted at all, and at the time of the backup job executing if any open files are present they will have to be skipped as files in that backup job run in that case (which may not be reasonable or feasible to have as a disabled backup job option in that case). You could also figure out how to get the files / plugin content on a volume that isn't larger than 64 TB, if that is at all feasible; if the backup job only has files but no plugins in the backup job selection, then the former would be easier to accomplish, as compared to if files + certain plugins, such as "Hyper-V", are in the backup job selection (that in that case you may need to create a separate "Hyper-V" backup job, and a separate other "files" backup job in that case to make that more feasible to do, as to not require to have the files on a volume that is under 64 TB in size, and to have the plugins on a volume that is under 64 TB in size, for a files + plugins backup job, to be able to make all of that work in the end with the "Open File Backup" backup job option enabled, which is a default).
If the drive letter (volume) that you are selecting files on, as part of the file backup selection "Backup" dialog, if one or all of the selected drive (volume) is larger than 64 TB in size, and if the "Open File Backup" (VSS) backup job option is enabled (which it will be by default) in it at the time of execution, if the backup job uses a "Local Backup" or "Cloud Backup" device type (that are both using the modern FPI backup engine in that case) then the backup job probably won't show any warnings or errors in it, and the example backup job log, when this problem is hit, will look like this in that case. In this case the backup job only contains a single "E:\Backups" folder (where the E: drive has a 72 TB volume size) in the backup job selection (not the entire E: drive volume), and both the full seed backup job run and the incremental backup job run end in "Success" status with no files backed up (when it should have backed up many files), and no warnings or errors will appear in this or in any other client produced log file for that matter (only the Event Viewer will show the actual issue, check the 2nd & 3rd screenshots below this 1st screenshot, to see what that looks like):
And, at that same time that the above example backup job had executed, after it ran into the VSS snapshot problem, due to the over 64 TB volume size in the backup job selection as being the issue (although we don't state an error that that is the case) , the "Event Viewer > Application" error, that is from the "Source: VSS", will be stated like this example screenshot shows, that matches the above backup job run, where the error event states : "Operation: Processing EndPrepareSnapshots", and above that as "Volume Shadow Copy Service error: Unexpected error DeviceIoControl(\\?\Volume{59ad6a4b-3142-43da-9f86-276795b94ea8} - 00000000000000FC,0x0053c008,000001DD7D00AC70,0,000001DD7D00BCA0,4096,[0]). hr = 0x80070057, The parameter is incorrect.", as seen in this 1st Event Viewer screenshot (1/2):
And the other VSS error (out of two total VSS errors) should look similar to this error text, "Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface. hr = 0x80070005, Access is denied.", as seen in this 2nd Event Viewer screenshot (2/2):
In the above first screenshot example of this problem, the one with the client backup log visible (*.summary.xml file), as well as the *.fpi.log file, as well as the "E: drive 72 TB" volume size text that is visible in notepad on the right there (all 3 of those items are visible in that same first screenshot), that affects a "Cloud Backup" device type (which is one of the modern FPI device type devices, which also includes the "Local Backup" device type as well) target device in that case, with the backup job only having the single E: drive in the selection (even if it was only a few files on the E: drive, or if it was the entire E: disk that was in the backup job selection as well) that caused the backup job to after it executed backup NO FILES, as the initial first seed backup run, where it should have certainly been able to backup many files, and that was all due to the fact that the E: drive itself was a 72 TB volume, which is over the 64 TB volume size for VSS ("Open File Backup" is the equivalent backup job option, which will be enabled by default, that will tell NovaBACKUP to backup the open files, and utilize VSS) to be able to work. If the "Open File Backup" backup job option were disabled at that point manually (and the backup job saved after that) then running the above backup job one more time would have worked fine, but that is because we would bypass taking a VSS snapshot of that E: drive that is in the backup job selection, that is the 72 TB volume in that case, and since the "Open File Backup" (enable VSS) backup job option was enabled on that example backup job run, that is what lead to this issue, even though the backup job completed with the "Success" status so it may be difficult to realize what is wrong in that case, which is the culprit here.
You can read more about that VSS limitation in this Microsoft.com article here (https://learn.microsoft.com/en-us/troubleshoot/windows-server/backup-and-storage/usability-limit-volume-shadow-copy-service, in the line of text in it that states "You run a backup operation on a volume that is larger than 64 TB that has a shadow copy enabled.".