I have a Highpoint RocketRAID 2720SGL, and I have a RAID-5 array with 3x 2TB drives, giving 4 TB of usable space. I'm adding an additional drive and I would like to move this to a 4x 2TB drive array giving 6 TB of usable space. Is this possible without copying all my data to another location and then rebuilding from the array scratch? I don't have enough room elsewhere to copy my data.
You tagged this as hardware-raid, but the RocketRAID 2720SGL is fakeraid, i.e. software RAID.
The instruction manual discusses Online Capacity Expansion and Online RAID Level Migration, though doesn't go into details. That's what you want to do. Add your fourth disk and use OCE. Of course, you'll want to make a full backup first, and be aware this process may take many hours.
Highpoint has a document which explicitly covers expanding the storage capacity.
Online Capacity Expansion (OCE) allows data to be expanded across new disks without taking the system offline. Storage can be added and introduced into the system through the HighPoint RAID management utility without the requirement to take down all of the storage to reconfigure the storage.