To write an image to an SD card with
sudo dd if=the.image.file.path of=/dev/sda bs=4M status=progress
The thing most likely to get confused about and screw up here is that the device node should not include a partition number -- e.g.,
/dev/sda1, which refers to the first partition on the
sda device. Instead you want the whole device, which is just
/dev/sda. If you use
sda1, the card won't work.
Pretty sure a card in a USB adapter will get labelled exactly that, unless you have other drives attached in which case it could be
/dev/sdb, etc. Have a look at
ls /dev with the adapter and card in. If
sda isn't there, it might have ended up as
mmcblk1 (but I don't think so). The trick with the partitions is that unless the card has been messed up or formatted in an unusual way, it will have at least one partition on it, meaning there will be an
sda and and
sda1 or a
mmcblk1p1. Again, you want the first one.
Just don't use
mmcblk0. That's the system card in the Pi.
The Retropie github refers you to the Pi Foundation page about creating a card with linux, which talks about
dd and device nodes some more. Their invocation is a bit different than mine but either one should be fine.