It depends on the keyboard but it is generally the key \ or one of the ones close to Enter.

@aussie-daredevil Then I recommend starting from scratch with the new disc, make sure it works, then transfer your files to it from your old drive - ROMs/artwork/etc. See the 2nd backup method explained in the docs - https://retropie.org.uk/docs/Updating-RetroPie/#making-a-backup-option-2.

@RossiPosse Rather than removing it, it always helps to share your solution with others. To that end, the first menu option in the raspi-config will expand the filesystem on the SD card.

@jdrassa Thanks, it worked!

@sigmanutz Ideally you'd look at the dimension of the 64Gb drive with df and note the number of total blocks of the / partition, then compare to the df output from your 128Gb card (also in blocks) and note the number of blocks used. But, just to make sure, just backup the biggest ROMs folder to your PC and you're sure to get a smaller image and the creation process will also be shorter. You can then move back the ROMs to the new system.

@mattrixk Makes sense. Thanks a lot for the info.