Hi
I own a 451+ with 4x3TB in raid 5, now with 4x4TB
I recently had a 3TB drive fail so decided to replace the drives with some new 4TB drives. I did the process of the replacing the drives one by one as per guide.
Yet finally done rebuilding it, the option to expand the volume is greyed out …
Where exactly do you see that option ? Are you using a static volume or volumes on a storage pool ?
OK, so the RAID device has the full capacity, if your backups are solid, you can try to manually grow your volume
Here is an old(ish) topic with step by step instructions on how to grow a stuck static volume
https://forum.qnap.com/viewtopic.php?t=173439
In your case it looks like md1 (the RAID itself) has already grown, so you should be able to skip this step)
Since this issue is related to data, we recommend that you open a support ticket so our Support team can assist you directly.
Thank you!