To format "flash" the internal memory:
Get the universal flasher
here....
Then...unpack the contents of the archive to the root of your SD card.
Plug the flash card into the PRS-505
Press 6
Wait for the PRS-505 to report "Reflashing was successful. Rebooting."
Take the flash card out of the PRS-505
Wait for the unit to reboot
The unit is now without operating system, now download the latest firmware from sony and update the unit to this firmware.
Enjoy