The root directory is always the same size regardless of the size of the SD

When flashing a debian image to a 64Gb SD card, the root directory always has a size of 3.5Gb. The same thing happened with the 8Gb SD card. How to transfer all remaining memory on the SD card to the root directory?

Hi Alex, what debian image are you flashing?
From this site i downloaded img: Debian 9.9 2019-08-03 4GB SD LXQT
Flashed my 64GB SD card. But when i use this SD card on beaglebone black there is always not enough free space on disk.

You’ll need to expand the partition using fdisk (or similar) and then expand the filesystem using resize2fs I believe.