A blade server is a type of server that is installed in a chassis or an enclosure that provides power, cooling, networking, and management features. The blade server does not have any cables attached to it, as it connects to the chassis through a midplane or a backplane. A blade server can save space, energy, and cost compared to other types of servers. Verified References: [Blade server], [Chassis]

The administrator should rebuild the volume to restore it after replacing the failed hard drive. A volume is a logical unit of storage that can span across multiple physical disks. A volume can be configured with different levels of RAID (Redundant Array of Independent Disks) to provide fault tolerance and performance enhancement. When a hard drive in a RAID volume fails, the data on that drive can be reconstructed from the remaining drives using parity or mirroring techniques. However, this process requires a new hard drive to replace the failed one and a rebuild operation to copy the data from the existing drives to the new one.
Rebuilding a volume can take a long time depending on the size and speed of the drives and the RAID level.

Explanation
The subnet mask is most likely containing an error that prevents the application server from communicating with the newly installed database server. The subnet mask is a binary number that defines how many bits of an IP address are used for the network portion and how many bits are used for the host portion. The subnet mask determines which devices belong to the same network or subnet and can communicate directly with each other without routing or switching devices. The subnet mask of the database server is 255.255.O.O, which means that all 32 bits of its IP address are used for the network portion and none for the host portion, which is invalid and makes it unreachable by any other device on any network or subnet. The subnet mask of the application server is 255.O.O.O, which means that only 8 bits of its IP address are used for the network portion and 24 bits are used for the host portion, which is also uncommon and makes it incompatible with most networks or subnets. References: [CompTIA Server+ Certification Exam Objectives], Domain 4.0: Networking, Objective
4.1: Given a scenario, configure network settings for servers.

Active-active is a configuration in which both nodes of a redundant system respond to service requests whenever possible. It can improve the performance, availability, and load balancing of the system by distributing the workload among the nodes. However, it also requires more synchronization and coordination between the nodes to avoid conflicts or errors. Verified References: [Active-active], [Redundant system]

The best practice for troubleshooting is to follow a logical and systematic process that involves identifying the problem, establishing a theory of probable cause, testing the theory, establishing a plan of action, implementing the solution, verifying functionality, and documenting findings. Since the problem occurred after a change in the server's BIOS, the most likely cause is that the change was incompatible or incorrect for the OS. Therefore, the next step should be to reverse the latest change made to the server and reboot to see if that fixes the issue. Reference: https://www.comptia.org/training/resources/exam-objectives/comptia-server-sk0-005-exam-objectives (Objective 4.3)