A command which may be of interest to make use of unused space on the SD card is resize2fs. This resizes the filesystem, and can be used while the filesystem is online (i.e. no need to unmount it).
First you have to use fdisk to resize the partition to fill the whole SD card. You can do this by doing:
Press “p” to show the current partitions and note down the details of the main one that RuneOS is on.
Delete it, pressing “d” and choosing the relevant partition number.
Press “n” to make a new partition, and “p” to select a primary partition. Enter the same start block that you wrote down above, and allow it to use the default end block (the last block available on the device).
Press “t” and enter the value “83” to mark it as a Linux partition.
Press “w” to write changes and exit. You may have to reboot after this for changes to be registered. This has resized the partition, but not yet the filesystem.
Next, use the resize2fs command in this way:
(or /dev/mmcblk0p1 if the main RuneOS partition was mmcblk0p1).
This will resize the filesystem to fill the whole partition, and you will have a lot more space available for stuff.