After playing with this PirateBox installation procedure and being unlucky multiple time, I can deliver some information on how the system is built. which can help you to understand faster and avoid some troubles.
Once flashed with OpenWRT the TL-MR3020 system have different layers, the first one is a readonly filesystem loaded in /rom and mapped to / . This file system contains all the needed elements to run the OpenWRT with a size of 4MB. This is what you flash during the installation process.
Another filesystem mounted in /overlay and also mapped to / is used as a second layer to have a RW space on top of the /rom filesystem content. It allows to create and modify configuration files, to add application not part of the initial imagefile. The size of this filesystem is really small with 1.5MB and do not allow to install anything on the system. The risk is to fill the entire space and no being able to use correctly the device.
To work correctly, the PirateBox adds another storage space to extends filesystem. This layer is installed on the USB Stick you are providing during the installation process. An image file is created on the memory stick and mounted as /mnt/ext. For this reason, if you change the usb stick you must copy all the files to the new stick to get the PirateBox software installed and work. The size of this extension is not especially limited but created initially with a specified size of 19MB. This device is mapped to loop0.
Matthias' edit: removed spam links
Once flashed with OpenWRT the TL-MR3020 system have different layers, the first one is a readonly filesystem loaded in /rom and mapped to / . This file system contains all the needed elements to run the OpenWRT with a size of 4MB. This is what you flash during the installation process.
Another filesystem mounted in /overlay and also mapped to / is used as a second layer to have a RW space on top of the /rom filesystem content. It allows to create and modify configuration files, to add application not part of the initial imagefile. The size of this filesystem is really small with 1.5MB and do not allow to install anything on the system. The risk is to fill the entire space and no being able to use correctly the device.
To work correctly, the PirateBox adds another storage space to extends filesystem. This layer is installed on the USB Stick you are providing during the installation process. An image file is created on the memory stick and mounted as /mnt/ext. For this reason, if you change the usb stick you must copy all the files to the new stick to get the PirateBox software installed and work. The size of this extension is not especially limited but created initially with a specified size of 19MB. This device is mapped to loop0.
Matthias' edit: removed spam links