Note that, this procedures is applicable for size upgrade (not down size!!!)
I have a Synology DS214play running with two 2TB HDD. Now I need to upgrade them to 6TB. The steps are very simple.
*** (the following figures are captured after upgrade, so ignore the HDD size concerned)
1. Go to Storage Manager ->Volume and make sure it is Healthy (not repairing/degraded!!!)
(before upgrade the my Total Capacity was 1.81 TB)
2. Go to Storage Manager -> HDD/SDD and make sure they are Healthy
(before upgrade both drive capacity were 1.81 TB)
3. Identify which is physically Drive 1 on your NAS.
Then you can see your Drive 1 LED color becomes Orange
4. Select Drive 1 and Action -> Deactivate (you need to input admin password to do so)
5. Unplug the Drive 1. (make sure it is the correct drive located in step 3). You will hear beep for RAID degraded notification.
6. Replace Drive 1 with a 6TB HDD and plug it back to the bay.
7. Go to Storage Manager->Storage Pool -> Action -> Repair
Then Click Apply on next screen.
And you will see something like this.
(during repairing of Drive 1, you will see Drive 1 becomes 5.46TB and Drive 2 is still 1.81 TB)
After a while, it will start the synchronization. (It takes hours or even days to complete depending on your data size)
After hours repairing, the volume status will back to Healthy.
(after repairing of Drive 1, you will see Total Capacity still being 1.81 TB)
8. You can repeat step 1 to 7 for Drive 2 (BEFORE THIS, MAKE SURE YOUR Volume status is Healthy!!!)
After repairing of Drive 2 completion, you will see Total Capacity growing to 5.43 TB)