After setting up a RAID 5 array, the next step is to make the storage space usable by the operating system.
This involves partitioning the array to define how the storage space is organized and then formatting it with a file system that the operating system can understand, thus making it possible to copy files to the array.
Decreasing the number of drives, rebuilding the array, or checking the hot spare status would not address the issue of the array not being ready to store files.
References:
* CompTIA A+ Core 1 Exam Objectives Section 3.3: Given a scenario, select and install storage devices.