Wojtek,
The easiest way to make a backup copy of the OS in the RPi4 is to insert a USB stick or USB uSD card (card reader with card) into one of the USB ports and copy the contents of the working OS to it. To do this, go to the raspberry icon and Accessories and then start SD Card Copier.
Select the source Device, then the Target Device and depending on the read/write speed you will get a copy in 30-10 minutes.
Pay attention to which is the source and which is the target! This is a Debian copy operation.
The OS in the uSD card holder can be copied to an external card or USB stick.
(If you will use an external USB stick as the OS, the same copy operation must be performed for the backup.)
Have a USB stick or uSD card of sufficient size and good quality, because then you can make a copy.
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Gyula HA3HZ
HA3HZ