Applying security updates is one of the common operating system hardening methods that can help protect the OS from cyberattacks and vulnerabilities. Security updates are released by the OS developer to fix bugs, patch security holes, and improve performance. By installing the latest updates, the server administrator can ensure that the OS is up to date and secure12.

Explanation
RAID 1 (mirroring) is a RAID configuration that should be used to provide redundancy for the OS on a server that has two hard drives on which to install the OS. RAID 1 (mirroring) is a configuration that duplicates data across two or more drives. It provides fault tolerance and improves read performance, but reduces storage capacity by half. If one drive fails in RAID 1, the other drive can continue to operate without data loss or system downtime. RAID 0 (striping) is a configuration that splits data across two or more drives without parity or redundancy. It improves performance but offers no fault tolerance. If one drive fails in RAID 0, all data is lost and the system cannot boot. RAID 5 (striping with parity) is a configuration that stripes data across three or more drives with parity information. It provides fault tolerance and improves performance, but reduces storage capacity by one drive's worth of space. RAID 5 can tolerate one drive failure without data loss, but not two or more. RAID 6 (striping with double parity) is a configuration that stripes data across four or more drives with double parity information. It provides fault tolerance and improves performance, but reduces storage capacity by two drives' worth of space. RAID 6 can tolerate two drive failures without data loss, but not three or more. References:
https://www.howtogeek.com/199068/how-to-upgrade-your-existing-hard-drive-in-under-an-hour/

The RAID level that should be implemented to maximize usable capacity without requiring redundancy is RAID 0. RAID (Redundant Array of Independent Disks) is a technology that combines multiple physical disks into a logical unit that provides improved performance, reliability, or both. RAID 0 is a RAID level that splits data evenly across two or more disks without parity or mirroring. RAID 0 does not provide any redundancy or fault tolerance, but it increases usable capacity and performance by allowing parallel read and write operations.

The per-concurrent user licensing model is a type of licensing model that restricts the number of active sessions or connections to a software application at any given time. This means that multiple users can share the same license, as long as they do not access the application simultaneously. This model is often used for applications that are accessed intermittently or for a short duration by different users, such as remote access software, web-based applications, or testing tools12.