To increase the amount of storage for IBM Security QRadar and move data to an offboard storage device specifically for /store/ariel, the appropriate method is to use iSCSI for external storage. iSCSI (Internet Small Computer Systems Interface) is a robust method that allows for the management and expansion of storage over a network, making it suitable for the needs of /store/ariel in a QRadar environment. This approach ensures reliability and seamless integration with the existing system.