Most RAID systems require that all disks in an array are the same size to achieve this. That requirement can be very limiting when trying to expand an array over time. An ideal filesystem will make efficient use of whatever storage you give it. Error-correcting. Modern hard drives hold a...
You can determine how much is usable across all devices using the btrfs disk space calculator https://carfax.org.uk/btrfs-usage/?c=2&slo=1&shi=1&p=0&dg=1&d=4&d=3&d=3&d=3. With 3+3+3+4 TiB, 1TiB will not be usable with a RAID1 profile. With 4x3TiB + 1x4TiB all ...