Eventually you will come to the point where you’ll need more storage. I’m running TrueNAS and there are 3 main ways of increasing storage capacity. You could replace every disc in the pool to a larger size, one disc at a time resilvering each disc as you go and once all discs are replaced the pool will have grown in size. The second option is to add another vdev to the pool and last create another pool. I’m using a Dell R320 with 8 3.5″ bays and have no available slots in my server for more discs so after some time looking at the options I decided to go with 2 x EMC DAE. DAE stands for Disk Array Enclosure. There are two different types, one with 25 2.5″ slots and the one I’ve chosen to use, the KTN-STL3 which has 15 3.5″ slots with SAS 2 controllers.