I recently had the opportunity to replace a bunch of ageing 2TB drives in my OpenMediaVault NAS with some 3TB drives. I run it in a 6+2 RAID-6 configuration (yes, I know, RAID is dead). I was a bit cheeky and replaced 2 drives at a time and let it rebuild. This isn’t something I recommend you do in the real world. Everything came up clean after the drives were replaced. I even got to modify the mdadm.conf file again to tell it I had 0 spares. The problem was that the size of the filesystem in OpenMediaVault was the same as it was before. When you click on Grow it expects you to be adding drives. So, you can grow the filesystem, but you need to expand the device to fill the drives. I recommend taking a backup before you do this. And I unmounted my shares before I did this too.
If you’re using a bitmap, you’ll need to remove it first.
mdadm --grow /dev/md0 --bitmap none mdadm --grow /dev/md0 --size max mdadm --wait /dev/md0 mdadm --grow /dev/md0 --bitmap internal
In this example, /dev/md0 is the device you want to grow. It’s likely that your device is called /dev/md0. Note, also, that this will take some time to complete. The next step is to expand the filesystem to fit the RAID device. It’s a good idea to run a filesystem check before you do this.
Then it’s time to resize (assuming you had no problems in the last step).
You should then be able to remount the device and see the additional capacity. Big thanks to kernel.org for having some useful instructions here.