You will have to reduce the size of whatever is stored on the
md0 array first. Unfortunately you give very little information on that.
- If there is a plain filesystem directly on
/dev/md0 then it depends on the filesystem type how you can reduce its size, if at all possible.
- If there is an LVM physical array on
/dev/md0 then you first have to reduce the size of that, which in turn may mean you here also have to reduce a filesystem, then reduce the logical volume, then reduce the volume group, then the physical volume.
As you're trying to add disks to a RAID5 consisting of (slightly larger) 2TB disks, it might be easiest to first assemble a RAID5 with the 2 new disks, pass
missing as the name of the third disk which will create a RAID5 with one disk missing. Now copy the data over from the old RAID5 to the new RAID5. Disconnect the old RAID5 disks and verify all your data is available on the new RAID5. Now you can reconnect the old RAID5 disks, use
mdadm --zero-superblock on the old component disks (perhaps you might need to do
mdadm --stop /dev/md0 first), this wipes any information about the old RAID5. Now you can add the disks to the new RAID5.