# Understanding physical drive capacity and unusable capacity

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

For example, if you group three 1 GB drives and one 2 GB drive into an array, the total capacity of the array is 1 GB times 4, or 4 GB, not the 5 GB physically available. The remaining space on the 2 GB drive is unusable capacity.

For the ServeRAID-8i controller and ServeRAID-7t controller, the remaining space is usable capacity. That is, you can use the remaining space to define another logical drive; see Example: Usable and unusable capacity.

Conversely, if you add a smaller drive to an array of larger drives, such as a 1 GB drive to a group containing three 2 GB drives, the total capacity of that array is 4 GB, not the 7 GB physically available. Therefore, the optimal way to create arrays is to use physical drives that have the same capacity.

A hot-spare drive also must be at least as large as the smallest drive in the array.

## More information

- Example: Usable and unusable capacity
- Example: Total disk capacity
- Understanding creating basic arrays

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