SystemRescueCd is based on a read-only compressed filesystem. All the files of the system are stored in a squashfs filesystem which is stored in sysrcd.dat. As a consequence, the changes you make on the system are not saved, and they are lost when you reboot (except what you do on the other filesystems that you may have mounted).
The main filesystem in SystemRescueCd is aufs, so changes on system files are allowed and stored in memory. It allows you to change a system file, for instance you can replace a program with your own version.
Users who wants to keep their changes in the system files can make a customized SystemRescueCd disc. It is very convenient when you want to add new programs to the system, but it is not comfortable if you often have to change files in the system. You do not want to make a new customized version everyday.
That is why SystemRescueCd provides the backing-store feature. A backing-store is a loopback filesystem stored on an USB-stick or on an hard drive, which contains all the files of the system that have been changed. The modifications are saved to the backing-store every time you edit a file, when you create a new directory, or when the system writes or deletes a file for any other reason. As a consequence, it allows you to keep your configuration changes: you can add your bookmarks and extensions to Firefox, and they will still be there when you reboot SystemRescueCd, as long as the same backing-store is loaded.
A backing-store is a mere ext3 loopback filesystem. You can create a new backing store or grow an existing one using sysresccd-backstore from SystemRescueCd. The system automatically loads backing-stores named sysrcd.bs which are saved at the root of a writeable filesystem. In other words, you do not have to specify any boot option if you create a backing-store in the default location, and the system scans all the hard-disks partitions and usb-sticks at boot time. You can disable this behaviour by adding the following boot option: backstore=off. Unfortunately backing-stores cannot be copied on a cdrom / dvdrom, since discs are mounted read-only, and backing-stores can only be loaded in the early stage of the boot process.
The backing-store file does not have to be on the same partition as sysresccd. It means you can have the main SystemRescueCd files installed on an USB-stick, and you can have a backing-store saved on another USB-stick. Anyway, the most convenient way o use it is to save both the main SystemRescueCd files and the backing-store on the same USB-stick.
Here is how to create and use a backing store:
The default place where to save the backing-store is the root of a partition in a file named sysrcd.bs. You are free to save the backing-store in an alternate location or with another name. In that case the file will not be automatically found by SystemRescueCd unless you specify the path using the following boot option: backstore=/path/to/mysysrcd.bs. By default SystemRescueCd scans all removable disks (eg: usb sticks) at boot time. If your backing-store file is on an hard-disk partition, you will have to boot with the option backstore=alldev so that it scans all block devices, not just removable ones. You can specify two options in the same time, for instance backstore=alldev,/path/to/mysysrcd.bs will search any file named /path/to/mysysrcd.bs on any block device of your computer.
You can also grow an existing backing-store with no data loss: