I have 3 3TB drives, and have raid5ed them together. I would expect to get a resulting device around 6TB. The command I used: mdadm --create md0 --level=5 --raid-devices=3 /dev/sda1 /dev/sdb1 ...
I have a 5-disk Intel RAID 5 along with a 6th boot disk with /, /boot, and swap. What I was planning to do was mount the Intel RAID partitions (which I've added with fdisk) so that the 6th disk ...