you can lock files, see below from univew nvr user guide, i use this feature with the clipping function, just rember to backup to usb or unlock the file to freeup the disk spce
Use this function to lock a recording file so it will not be overwritten.
Click for the recording you want to locking the playback window.
b.Click and then click the Locked Filetab to view the locked file.
To unlock afile, click , and the icon changes to .
To back up a file, select the file and then click Backup.