Upsizing hard drives as pool nearly full in General Posted June 13, 2020 But then you could: 1. Attach USB disk and add to the Pool; 2. Remove one HDD from Pool (through GUI); 3. Replace that HDD with a big one and add that new one. 4. Remove USB HDD from Pool (through GUI); 5. Depending on size and duplication, you can do the other three all at once or one by one.