You have 'one' logical volume, in case your DS209 is configured with a RAID1 mirror set of two hard disks. Just create a new shared folder and it will be created on your disk array. Create the new user for the FTP access and give it access to this share.
DS107 DSM 3.1-1636; 1x WD10EADS GP; IP-cameras Panasonic BL-C1 and BL-C101; APC Back-UPS ES 700; USB disk for data backup;
DS209 DSM 3.1-1594; 2x WD15EADS GP 1500.299.234.816 bytes SATA II 32 MB cache in RAID 1; American Power Conversion RS 800; two USB disks for data backup; Synology Remote and Creative SB Connect; Netgear GS105 Gigabit Ethernet Switch; aLaptop, iMac, uPC; a common modem/router/firewall; a dose of common sense ;)