Capacity overhead refers to the storage capacity consumed by parity data or mirrored copies rather than usable data.

• RAID 60: Combines striping with double parity. Each parity group (RAID 6 set) consumes the capacity of 2 drives for parity. With two parity groups, the total overhead is $2 + 2 = \textbf{4 drives}$.
• RAID 50: Combines striping with single parity. Each parity group (RAID 5 set) consumes the capacity of 1 drive for parity. With two parity groups, the total overhead is $1 + 1 = \textbf{2 drives}$.
• RAID 10: Uses mirrored pairs. With 4 drives, half the raw capacity is used for the mirror, resulting in an overhead of 2 drives.
• RAID 1 ADM: Advanced Data Mirroring creates three drive copies of the data. With 3 drives, only 1 drive's worth of capacity is usable, resulting in an overhead of 2 drives.

HPE Smart Array SR Gen10 Configuration Guide, Part Number: 882375-002.Section: RAID Levels. Describes RAID 1 ADM (Advanced Data Mirroring) as using three drives for the same data (Drive 1 + Drive 2 + Drive 3 = 1 Usable), RAID 10 as using $n/2$ capacity, and the parity overheads for RAID 50/60.Dell PowerEdge RAID Controller (PERC) Series 9 User's Guide.Chapter: Understanding RAID. Defines RAID 50 and RAID 60 parity overhead calculations per span (parity group).SNIA (Storage Networking Industry Association) Dictionary of Storage Networking Terminology.Definitions for RAID Level 6, RAID Level 5, and Parity Overhead.