A Dell PowerEdge R750 shows a sudden reduction in usable capacity on a RAID 6...

When an administrator replaces a degraded physical drive within an existing RAID 6 array with a drive of a larger capacity, the usable storage space of the virtual disk layout remains bound to its original values. In hardware storage configurations managed by a PowerEdge RAID Controller (PERC), the maximum storage contribution of any individual member disk within a specific array group is strictly governed by the footprint of the smallest operational drive initialized during the array's creation. Consequently, even though the new disk successfully initializes, passes validation, and registers an 'Optimal' or 'Ready' status via the iDRAC management engine, the excess capacity residing on that larger drive remains unallocated and unusable within the current parity volume configuration. To exploit the additional raw capacity, every single participant drive in that specific RAID group must be sequentially upgraded to matching larger sizes, allowing the controller to successfully execute a virtual disk extension or Online Capacity Expansion (OCE) process across the expanded array boundaries.

Study Guide References: Server Maintenance; PowerEdge RAID Controller (PERC) Layouts; Storage Array Rebuild Mechanics.