Managing RAID and Mixed-Capacity Drives

Modern storage systems rarely grow in a perfectly planned way. A server may start with a few small disks and later be expanded with much larger ones. This creates a common challenge: how to efficiently use drives of different sizes while keeping performance.

This article explains the problem, reviews modern solutions, and highlights important technical risks you should understand before choosing an approach.

 

How to set up RAID with drives of different sizes

Modern storage platforms no longer treat RAID as a single, rigid technology. To set up RAID with different drives use flexible RAID or parity-based systems that allow mixed-size drives, such as SHR, Unraid, or Storage Spaces. There are several approaches to keep data safe when using drives of different sizes: vendor-managed RAID setups, OS-managed RAID,  and Parity-based storage with separate disks. 

Synology Hybrid RAID (SHR)

Synology Hybrid RAID was designed specifically for environments where disks are added over time. Instead of forcing all drives into a single rigid layout, SHR internally divides disks into smaller logical segments and combines them into multiple RAID groups. This allows the system to use the extra space on larger disks once smaller disks are fully utilized.

For users, the complexity is hidden. Expansion is simple, space utilization is efficient, and redundancy is maintained automatically. The downside is that SHR is proprietary and only available on Synology systems, which limits portability and transparency.

Unraid

Unraid takes a radically different approach. Rather than striping data across all disks, each data drive remains independent and uses its own filesystem. One or two dedicated parity disks are used to protect against failures.

This design allows drives of almost any size to coexist in the same system. The only strict requirement is that the parity disk must be at least as large as the largest data disk. Expansion is easy, and a failed disk only affects the data stored on that disk.

The trade-off is performance. Because data is not striped, write speeds are generally lower than traditional RAID. For workloads like media storage or backups, this is usually acceptable, but it may not be suitable for databases or heavy virtual machine use.

Windows Storage Spaces

Windows Storage Spaces abstracts physical disks into a storage pool and applies redundancy at the virtual disk level rather than directly on the drives themselves. This allows mixed-capacity disks to be combined more flexibly than classic RAID.

The system supports mirroring and parity layouts, but performance depends heavily on proper configuration. In particular, aligning the interleave size with the NTFS cluster size can significantly reduce parity-related write slowdowns. When configured correctly, Storage Spaces can be a solid option for Windows-based servers, though parity writes remain slower compared to hardware RAID.

The Challenge of Mismatched Drive Sizes

Traditional RAID systems, especially hardware RAID controllers, are designed around symmetry. They apply what is often called the “smallest drive rule,” meaning every disk in the array is treated as if it were the size of the smallest member. If you mix a 2 TB drive with an 8 TB drive, only 2 TB of the larger disk will be used. The remaining space is simply unavailable.

This approach makes RAID predictable and reliable, but it can waste a significant amount of storage. Users must decide whether preserving redundancy is worth sacrificing usable capacity. As disk sizes increase, this trade-off becomes more painful, especially when large drives are expensive.

Performance is another concern. RAID arrays tend to operate at the speed of the slowest disk. Differences in rotational speed, cache size, or disk technology can reduce throughput for the entire array. During rebuilds, which already take much longer with modern high-capacity drives, these performance gaps become even more noticeable and increase risk. 

 

Designing RAID for Real-World Disk Growth

Mixing drive sizes is no longer an edge case—it is the normal evolution of real-world storage systems. While traditional RAID remains reliable, its rigid design often leads to wasted capacity and longer, riskier rebuilds as disks grow larger. Modern software-defined approaches such as Synology Hybrid RAID, Unraid, and Windows Storage Spaces address these limitations by trading strict symmetry for flexibility and smarter capacity utilization. Each option comes with its own balance of performance, transparency, and portability, making workload and platform choice critical. Ultimately, the safest and most efficient solution is not the one that maximizes raw disk space, but the one that aligns with how your data is accessed, how often your system grows, and how much operational risk you are willing to manage over time.