Backup disc doubles discspace?

I just bough two Toshiba 500GB hard drives. I transferred 18GB of pictures and events(FCPX) to the one disc. I used the other one as copy(copied the events inside FCPX). The backup disc now tells me that it has 40 GB of used space and the original 18 GB. How is that possible?

I formatted the discs before started using them.

How are the drives each formatted?

Disc utility and exFat both of them.

Can't use ExFAT with FCP X.
Format them as Mac OS Extended (Journaled).

What? Weird, I'm using ExFAT on every drives. But I will go for Mac OS Extended in the future.

Will I notice any differences from ExFAT to Mac OS E?

Can I drag all my event and project folders to a temporarily folder in Finder, format the drive to Mac OS Extended and then drag it back to the drive? Or do I have to do everything inside FCPX?

You will notice much more stability and you should see some speed improvement with a "properly" formatted drive. FCP X will work with ExFAT, but it won't be fast, it won't be stable, and could cause some issues.

Yes, you can copy in the Finder if you want.

That's good to hear!

So moving the folders inside Finder won't cause any trouble, like if I copy event folders to a backup drive outside FCPX?

That's fine. Just remember FCP X, with Finder copied/moved Events and Projects, can only read one copy at a time. So move to backup, format drive, move back to main drive, then either take backup drive offline, or delete the Events/Projects from the backup drive. Each Event and Project is an independent database, with it's own UUID. When you copy inside FCP X, it gives copies their own UUID. When you copy in the Finder, that UUID is duplicated, so you have two with the same UUID, which FCP X really gets upset about.

I'm not sure if it's correct, but it feels like rendering time has gone down after I formatted my drive.

Thank you for your help, Ben!

Render times should be better with a native format drive. No technical way for it to slow down.

Just installed my 2x8 GB. Rendering is faster! FCPX is also function better. No more lagging when do stuff.

In my upgrading spirit, will FCPX work even better with a internal SSD? Since my events and project files is on the external, will it be better to upgrade to a internal or external SSD?

Internal