You need to make a backup of halo 2 to your F drive, then put the replacement maps in the correct folder of your halo2 hdd backup, you can't ftp files to a game dvd, it's not re-writable, especially when it's in the XBOX.
Open a file explorer, copy halo2 to your f drive, you may need an upgraded one for this game 'cos it's big. Then ftp the maps to your f drive, delete the default maps, replace them with your modded maps. Launch halo2 from your f drive.
If this helps please add to my rep.