If your Wi-Fi network, or power to the Sync Module 2 goes out, clips are not saved to your USB drive. If the clip was partway through download when power went out, that file is lost. However...
If the system is using Cloud Storage, when power and Wi-Fi come back on, the Sync Module 2 will attempt to download any clips that did not save completely.
If any clips were auto-deleted from Cloud Storage during the outage, they are not present to be in the backup.
If the system was using Local Storage to record from cameras when the network or power stopped, then partially saved clip is lost when the Wi-Fi or power stops functioning.
If the Safe Eject option was not used prior to removal of the USB drive, the contents could be corrupted. The following message in Windows might appear if the drive is inserted into a computer:
Mac OS computers will show a similar message: "Disk Not Ejected Properly".
You can safely chose "Scan and fix" or "Continue without scanning" which will not cause any harm to your files or storage device. It is possible the files were corrupted however, and a scan and fix might not be successful.
Do not remove the USB drive when the green icon indicates a device is Connected.
Always tap the Safe Eject button before removing the local storage USB drive.