Hi,
I read that it is recommended to have a SD Card of the double capacity of the original one for auto backup.
I don’t understand why …
In this case, in case of crash of the first card, I would replace it by the second one … and I’ll have to provide one of four time the capacity of the original one (then eight, and so one 
).
Because Amanda doesn’t just make a clone of the SD card. It takes incremental backups. You can go back to a specific point in time, not just a duplicate of the most recent files. To handle that requires more space than the original.
So you don’t have to double to restore from the backup. You get a new SD card the same size as the original, install stock openHABian on it. Set up Amanda and then restore.
Thank you for your quick answer.
In this case, I understand.
I thought the “backup card” would be bootable for a fast switch/replacement. It is not the case ?
If you use any imaging software to make a 1 to 1 copy that can be booted there is a problem if the destination card is smaller then the source card. To fully explain this you need to look at how SD cards and many flash devices are mass produced. The flash is made in bulk and installed into a IC, then a tiny ARM processor is also fitted into the storage device. The flash gets tested and if a small part of the flash fails the testing process, instead of throwing the lot in the bin, they just mark the bad area as bad and still sell the product. This means if you buy two SD cards marked as 32gb, one of them may be a few kB smaller than the other. This poses an issue when you try to copy onto a card that is smaller in capacity despite having the same size written on the outside of the device. The answer is either you have to resize the partitions before the image is taken or you use the easier way forward and just buy a bigger card to restore onto.
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