The fastest way to bring a corrupted data drive back online with the least disruption in a VMware environment is through Instant Disk Recovery. This feature allows the administrator to quickly restore the specific affected disk from a backup, minimizing downtime and impact on business operations.References: Veeam Backup & Replication User Guide, Veeam Instant Disk Recovery Guide

In Veeam replication, the function that ensures changes made during a failover are copied back to the original production VM is Fallback. This process involves reversing the replication direction after a failover, ensuring that any changes made while running on the replica are synchronized back to the original VM.References:Veeam Backup & Replication User Guide, Veeam Replication Fallback Guide

To solve the space issue while enabling GFS retention, the engineer should add an Object Storage as a Capacity tier to the Scale-Out Backup Repository (SOBR) and set the policy to move backups that are older than 14 days. This approach allows for efficient use of local NAS storage for short-term retention and leverages object storage for long-term archival purposes.References:Veeam Backup & Replication User Guide, Veeam SOBR Configuration Guide

It is recommended to install Veeam ONE before Veeam Backup & Replication because Veeam ONE can potentially identify infrastructure issues that might affect backup operations before they are performed. This proactive approach ensures that the environment is optimized for successful backup and replication tasks.