A Dell PowerEdge R750 shows a sudden reduction in usable capacity on a RAID 6 virtual disk after replacing a failed drive with a larger one. The OS and iDRAC both report the VD as “Ready’" and “Optimal" but usable capacity remains unchanged. What is the cause of this behavior?

A fundamental principle of standard RAID arrays is that the capacity of a virtual disk is determined by the size of the smallest physical disk in the array. When a failed drive is replaced with a larger one, the RAID controller rebuilds the data onto the new drive but only uses a portion of its space equal to the size of the other drives. The extra capacity on the larger replacement disk remains unused. The virtual disk's size does not automatically expand; it remains constrained by the original, smallest member disk's capacity.

A. The virtual disk status is "Optimal," which indicates a successful rebuild. If the drive were a hot spare, the array would not be optimal.
C. The status is "Optimal," which directly contradicts the idea of a failed rebuild or a degraded/reduced capacity mode.
D. Virtual disks do not expand automatically upon drive replacement. This requires a separate, explicit Online Capacity Expansion (OCE) operation.

1. Dell PowerEdge RAID Controller (PERC) 11 User’s Guide. Section: "Replacing a failed physical disk". The documentation notes, "If you replace a physical disk with a larger physical disk, the extra space on the physical disk is not used."

2. Dell PowerEdge RAID Controller H755N PERC 11 User's Guide, Page 41, Section: "Replacing a defunct physical disk". This section explains the rebuild process and implicitly confirms that the array geometry is based on existing member disks. It does not mention any automatic capacity change.

3. Arpaci-Dusseau, R. H., & Arpaci-Dusseau, A. C. (2018). Operating Systems: Three Easy Pieces. Arpaci-Dusseau Books. Chapter 39: "Redundant Arrays of Inexpensive Disks (RAIDs)". The chapter explains the basic principles of RAID, including how capacity is calculated based on member disk sizes, which supports the concept that the smallest disk dictates the usable space per disk.