If you use software-raid you can use striping. Basically, divide up each disk into 1TB partitions, then RAID the 1TB partitions together. Synology uses this fo

If you use software-raid you can use striping. Basically, divide up each disk into 1TB partitions, then RAID the 1TB partitions together. Synology uses this for their Hybrid RAID. Obviously care needs to be taken to not confuse partitions but this lets you use disks of differing sizes. Unraid also offers RAID with differing sizes for disks, it has quite different failure modes though but it's been fairly reliable for me.