I signed up to post this, as I could not find any clear information regarding what to expect if you need to increase your disk size, and some of what I read on this forum, and also in other tubes of the Internets was downright wrong.
TL;DR:
You can replace smaller disks in a RAID1 with larger disks, and then resize the RAID partition without losing data. It takes a LOOOOOOOOONG time.
Here is the scenario that recently faced me:
I had reached the limit of my RAID 1 setup with two 2TB disks. I had about 100GB left.
Here is a synopsis of what I did:
Removed one of the 2TB disks and replaced it with a new, out of the box, WD 4TB disk. When I logged in to the web dashboard, the system informed me that one of the disks was corrupt, and that the system was rebuilding. It took about a day, but needed no further input from me, until it reported that the system had been rebuilt.
The disks (on2 2TB, and one 4TB) were correctly reported, but the space available still showed, of course, only 100GB.
I repeated the process with the second disk, and this time things only took about 8 hours or so.
The system now showed 2 4TB disks, but the space remained 2TB with only 100 GB free.
With some trepidation, I entered the RAID editing page, and there _clicked on the button to increase the available spac_e. I increased it to the maximum, 4TB.
It took another 24 hours to resize, but at the end of it all, I was greeted with a dashboard that showed 2 happy 4TB drives, and 2TB free.
I use Chronosync to automatically back my system up every day, and the scheduler resumed without as much as a hiccup.
I hope this helps some other poor sod like me, who couldn’t find anything definitive to refer to.
Steve