# Example: Usable and unusable capacity

Physical drive capacities influence the way you create arrays. Drives in an array can be of different capacities (1 GB, or 2 GB, for example), but RAID controllers treat them as if they all have the capacity of the smallest physical drive.

For example, if you group two 2 GB drives and one 3 GB drive into an array, the usable capacity of the array is 2 GB times 3, or 6 GB, not the 7 GB physically available. The 7 GB is the total disk capacity. In the following diagram, usable capacity is labeled as 1 and unusable capacity is labeled as 2.

2 GB | 2 GB | 3 GB

Similarly, if you group three 2 GB drives and 1 GB drive into an array, the usable capacity of that array is 4 GB, not the 7 GB physically available. The 7 GB is the total disk capacity. The remaining capacity left on the three 2 GB drive is unusable capacity.

The optimal way to create arrays is to use physical drives that have the same capacity. Doing so avoids unusable capacity.

## More information

- Example: Total disk capacity
- Understanding physical drive capacity and unusable capacity

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