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Tatjana - you might want to consider using a folder and file synchronize tool to mirror the libraries to the external usb. I have about 1.3TB of data on my internal 2TB SATA drive, takes a few minutes to mirror the changes to a usb at the end of the day, most of which is me eyeballing the preview to make sure there are no surprises.
This is a fairly comprehensive list of available alternatives ==>>
Comparison of file synchronization software
BR