How can I gain the 'true' space on this micro sd?
I recommend you insert it directly into a sd card reader attached to your Mac and backup the part of the drive you can access to your Mac's drive. Then open Disk Utility.app select the SD card reader in the left bar on the top level of the hierarchy (not the volume itself on the second level!), switch to the partition tab on the right hand, choose "1 Partition" from the "Partition Layout:" menu and click on "Options" beneath it. Choose "Master Boot Record", confirm the dialog with "OK" and click "Apply". Wait until it's finished, then check if the size of the volume has increased and if it has you can copy your files back.