piCorePlayer uses piCore which is predominately a RAM based system, so nearly everything is stored in RAM. If you modify or add files it is highly likely that you are doing the changes in RAM. So after a reboot your changes may disappear. To make changes persistent you need to do one of the following: add/modify files in your /home/tc or /opt directories, then do a backup—see piCorePlayer backup. The home and opt directories are already included in /opt/.
The “backup” on piCorePlayer is not a backup in the normal sense. As piCorePlayer is essentially a readonly RAM based system, it needs a mechanism to save the configuration files so the changes can be restored after a reboot. To achieve this, the piCore includes a program called filetool.sh that allows for a selective backup of files and directories that are listed in the file /opt/.filetool.lst—see Typical .filetool.lst The default backup file is /mnt/mmcblk0p2/tce/mydata.