Hi
if your aggr is hybrid, (SSD and HDD) - look for the articles for flash cache and confirm the limits for your configuration.
if the whole AGGR is just SSD you'll need to:
1. confirm the model number of the other SSD's (command in 7 mode sysconfig -a, in cdot - disk show -aggr <aggr nameL> -fields model)
2. confirm the aggr max size and disk count to your controller model against hwu.netapp.com
3. confirm the maximum and recommended amount of SSD in raid group (28) is not exceeded (command in 7 mode sysconfig -r and aggr show-status ). if it does - you might not be able to add just one disk to the aggr to add space.
https://docs.netapp.com/ontap-9/index.jsp?topic=%2Fcom.netapp.doc.dot-cm-psmg%2FGUID-EE30CDA9-095F-4DD1-9A29-BB9F0E55DB4F.html
once that all confirm - you can likely order the extra disk. Make sure you get it from a certified partner. Otherwise, you might risk the warranty/support of your array, and might get a refurbished or not proper compatible dire.
When you get the disk.
1. check if you need to increase the maxraidsize (in 7mode aggr options, in cdot - aggr show -aggr <aggr name> -fields maxraidsize)
2. insert the disk and confirm it's visible in disk show or sysconfig -r
3. assign it to the node where the aggr is on with disk assign command (7 mode and cdor)
4. add the disk to the aggr with aggr add-disks command.
5. run vol reallocation - to spread the workload across all the disks, and avoid having your disk deemed to be "cold"
Good luck
