Adding extra disk to Ultra 4 with 2 disks

[blackavenger]

I have recently upgraded my Ultra 4 with larger disks (2x4 TB WD Red) and also upgraded fw to OS 6.8.1-T106.
(this process was quite smooth except when I tried to install LogAnalyzer app and found out that the installer was broken and changed all files to uid/gid guest/guest&777. It took some time to recover).

I now have one X-RAID volume RAID 1, size 3.7 TB

Then I thought about a spare 3 TB WD Red. I inserted it in the 3rd disk bay expecting to get a larger volume. rn-expand.log shows:
auto_extend: Checking disk sdc...
Trying xraid-expand (tiered expansion)
Considering X-RAID auto-expansion for data
Checking if RAID disk sda is expandable...

Checking if RAID disk sdb is expandable...
0 disks expandable in data
Must I use a 4 TB disk to get data expansion?
Or do a factory reset with all disks in place?

(yes, I have backup and an UPS)

[StephenB]

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@blackavenger wrote:

Must I use a 4 TB disk to get data expansion?

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4 TB or larger to expand the array.

A factory reset with the 3 TB drive in place would also work (building the array from scratch).