Importing a new library backup

[Joliet Jake]

I have a fairly large library, and someone donated a library to me that's about double the size of mine.

I want to add their books as a new library (so I can switch between their library and mine).

First I tried "create/switch library" but when I directed Calibre to the folder holding the backups it said there is no library there.

Next I tried saying "import data" but I'm worried that will somehow overwrite my library rather than just create a second library. (I do have a backup to my library, but...) Has anyone ever done this? No amount of searching seems to reveal the answer. It's possible my Google search technique on this is bad.

I tried taking the plunge but it said the library was coded version 681578495 and I needed to upgrade. I did so but got the same error message.

calibre, version 5.36.0
ERROR: Not valid: The folder E:\Libraries\Documents\Ebooks\[name]\extreme-2tb-ssd-external\0-calibre-backup is not valid: The exported data in part-0009.calibre-data is not valid, version (681578495) is higher than maximum supported version. You might need to upgrade calibre first.

Traceback (most recent call last):
File "calibre\gui2\dialogs\exim.py", line 262, in select_import_folder
File "calibre\utils\exim.py", line 253, in __init__
ValueError: The exported data in part-0009.calibre-data is not valid, version (681578495) is higher than maximum supported version. You might need to upgrade calibre first.