Hi! I have mainly light server experience with linux, and I just got a new desktop to centralize all my data.

At the moment I have installed lubuntu on a SSD, and set up two 3TB drives for my data for RAID 1. It seems like it is working properly... except when I look in gparted the RAID array is using 42GB. I can't for the life of me figure out how there is any data there, where that data is even located, or what that data could be.

The OS install totals about 6GB. Navigating to where the array is mounted there was nothing there except the lost+found folder. I tried reformatting the array, it went to zero and then somehow found those 42GB again. I tried dropping about 9GB of video on the array. That adds to the total, and I can see that data where it should be, but the other 42GB are still unaccounted for.

I used this to build the array from the two hard drives.
# mdadm /dev/md0 --create --auto yes -l 1 -n 2 /dev/sdb1 /dev/sdc1
# mke2fs -t ext2 /dev/md0

It is md0 that has the mystery 42GB. Whereas sdb1 and sdb1 show in gparted that no data is being used. (There is also sda with the OS, doing it's thing all by itself I hope.)

If anyone has any guesses as to where that 42GB is coming from I would love to hear them, thanks in advance for any advice! Please ask if there are any details I should add that would help clear this up.

ext2? Why oh why would you use ext2? That's been dead and buried for over a decade.

Could you post the output of "df -h"?

Fair question re: ext2. ext2 because I was blindly following this: "The "super simple md" recipe" from https://wiki.ubuntu.com/Raid
(The ssd is currently ext4.) I only just figured out that mke2fs picked it's favorite default, in this case ext2. I haven't gone back to change it to ext4 yet.

df -h below shows the data I dropped on the array md0 totaling 8.8GB, but not the mystery 42GB I'm concerned about.

Filesystem Size Used Avail Use% Mounted on
udev 7.7G 0 7.7G 0% /dev
tmpfs 1.6G 9.5M 1.6G 1% /run
/dev/sda1 95G 3.6G 87G 5% /
tmpfs 7.7G 96K 7.7G 1% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 7.7G 0 7.7G 0% /sys/fs/cgroup
tmpfs 1.6G 12K 1.6G 1% /run/user/1000
/dev/md0 2.6T 8.8G 2.5T 1% /mnt/md0
/dev/sde1 58G 42G 16G 73% /mnt/64GB_usb

Here is a screenshot of gparted and the df -h read out side by side.
http://imgur.com/a/EuqX4

How do these drives attach to the motherboard? Not a fake raid controller I hope.

This is the computer I got.
http://www.microcenter.com/product/4...sktop_computer

After asking pretty much everyone in the store whether or not I would need a raid controller, they couldn't decide, and they also didn't have one in stock for me, so I bought this card off of amazon. I'm not sure how to identify a fake raid controller.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

When I got the computer open there were enough ports on the motherboard technically for all my drives, but at the moment the ssd and cd drive are connected directly to the motherboard and the two 3TB drives are connected to that card I bought, which is in turn installed on the mother board.

I just formatted the raid array (md0) to ext4, and as before, it deleted the data I had added to it as one would expect, but the mysterious 42GB remain according to gparted.
http://imgur.com/a/fpNti

You could try Partition->Check in gparted to make sure there's not some inconsistency. It's possible gparted shows root reserved space as "used" as well? If it's not there in df I wouldn't worry about it personally.

Ah, yes. That is probably it. I found this post where the person has the exact some concerns as me.
https://askubuntu.com/questions/4848...-discrepancies

I thought my only new variable was RAID but I guess I've never had such a large hard drive completely empty before, so I'd never scrutinized the available space so closely. I will stop worrying about it. Thanks yal for your input.