Other than adding a raid or sata cards.
TS can also do the followings.
Eg. current configurations.
1x 1TB
1x 2TB
1x 2TB
1x 4TB
Buy another 2x 4TB.
Then the 2x 4TB can replace the 1x 1TB & 1x 2TB & you will still have extra 5TB space to spare for many more years to come. And when you almost fill up the last 2TB, change that to another 4TB. Only when you finally fill up all 4 with 4TB drives, then only you consider getting a raid or sata card. By then, probably time to change motherboard or 6TB or 8TB drives will be out & you have more options again.
There are just many ways to solve a problem.
Think out of the box.