Let's say that you have utilized the maximum recommended storage space in your cluster, you have an empty slot for another drive and you want to fill it to expand the storage capacity.
 
There are different types of drives out there, each of our platforms has it's own specifications and requirements. 
 
To check which drives are compatible with your platform, choose your platform on this page and check out the data drives supported. 
 
Your node will be in one of the following configurations:
 
1) Hybrid: A mix of SSDs and HDDs. 
2) All Flash: can accept only SSDs and can contain only an even number of drives. 
3) SSD with NVMe: only certain drives slots can contain NVMe, check the system specification via the link above for more information. 
 
When adding more than one drive to a node, allow at least one minute between adding each of the drives.

Check out my colleague Jon Kohler’s reply here where he provided some good caveats for adding drives. Specifically, increasing the hot tier capacity on just one node can create a hot-spot and bring unexpected performance consequences.
 
For instructions and more information check out: NX Series Hardware Administration Guide: Adding a Drive
 
Let me know in the comments if you have any question.