1. Remove existing memory modules: Before adding new memory modules, the existing memory modules should be removed.
2. Add 16x 64GB DIMMs: The new DIMMs should be added, in this case is 16 DIMMs of 64GB each, which will expand the memory to the highest configuration available.
3. Shutdown the Database Server if running: The Database Server should be shutdown before adding new memory modules in order to prevent any possible data loss or corruption.

Failure groups provide redundancy in Oracle Exadata Database Machine X9M by ensuring that mirror copies of the data are written to different storage servers [1][2]. This prevents the potential for mirror copies to be written to the same server, which would cause a data loss if that server were to experience a physical disk failure. Furthermore, failure groups help to ensure that the ASM Flex instances are enabled on a maximum of two database servers or VMs per cluster, providing an extra layer of data protection. Additionally, Extended Redundancy mirrors data across data centers, providing the highest levels of data protection. Reference: Oracle Exadata Database Machine X9M Implementation Essentials [1][2], Chapter 5: Configuring Oracle ASM.

Once the physical disk has been replaced, you will need to create a new celldisk and griddisks on the replaced physical disk. After the griddisks have been dropped from the associated ASM diskgroups, you can then add the griddisks back into the ASM diskgroup they used to be a member of and rebalance the data on the effected griddisks. Finally, you should verify that the griddisks located on the physical disk have been successfully dropped from the associated ASM diskgroups.