https://toshelp.terra-master.com/web/#/en/storage_pool
The help documentation clearly states:
Migration requires adding disks to the TNAS, and the capacity of the newly added disks must be no less than the smallest disk in the current array.
To ensure your data security and the smooth operation of your array migration, please understand and adhere to the following capacity rules before adding a new disk.
As you can see from the calculator, when using a single 16TB hard drive, the available space is 16TB. Assuming a 16TB volume has already been partitioned, if a 2TB disk is added at this time, the available space shrinks to 2TB (this is due to the RAID logic). The new RAID space cannot accommodate the original 16TB of data, so we do not support choosing a smaller hard drive when expanding capacity.
When performing an online array migration (e.g., upgrading from Single mode to RAID 1), the capacity of any new disk you add must be greater than or equal to the minimum capacity of all disks in the current array.
The fundamental reason for this rule lies in the working mechanism of RAID arrays. When performing data mirroring or redundancy calculations, RAID arrays build a unified logical storage space based on the smallest capacity hard drive in the array. This is similar to the "barrel effect," where the effective capacity of the entire array depends on the shortest stave.
When you attempt to add a smaller capacity hard drive to an array composed of larger capacity hard drives, the system cannot accommodate the complete array data structure and file information within the smaller drive. Forcing this will result in data truncation, file system corruption, and ultimately irreversible data loss. Therefore, the system performs strict verification before operation; if the new disk's capacity does not meet the requirements, the migration will be automatically prevented to protect the security of your existing data.
For example: If your current array consists of a 16TB hard drive, then the new hard drive you use for the upgrade must be at least 16TB. Attempting to migrate using a 2TB hard drive will be considered non-compliant and rejected by the system.
We strongly recommend that you back up important data and choose hard drives with matching capacity and model before making any storage pool changes to ensure optimal compatibility and stability.
.