Problem when booting OS X from an external USB drive I would like to create a Fusion Drive from my SSD and HDD both installed inside my MacBook Pro 15" Mid 2012. I've read an article online which said that in order to do that, I'd need to boot Mavericks from an external USB drive. So I've picked a 64 GB USB stick and partitioned it in two, 8 GB for the installer and the rest for the operating system to install to. I've managed to create the USB installer of Mavericks using Unibeast, the installer boots without any problem, it installs normally but when I reboot I get a stop sign with a spinning wheel under it. How can I solve this problem?

The tutorial I'm trying to follow to create the Fusion drive is this one: Manually Create an Apple Fusion Drive on Mountain Lion/Mavericks/Yosemite



I need to create a Fusion drive from the 2 internal drives (240GB SSD and 1TB HDD). On the SSD I already have installed El Capitain and I used the HDD to store files. Now I've seen that you can merge this two in a single LogicalVolume and OS X will see it as if it were a Fusion Drive.
You do not need Unibeast (a tool for hackintoshes) to create an external installer. Can you confirm you're only using it to run the createinstallmedia command provided by apple as per this page?

Apart from that it seems you are confusing an external Mavericks installation with an external Mavericks installer. All you should have to do is double click the installer, straight from the App Store still laying in /Applications on your internal drive, then pick the external volume as a target (wipe the current ones and make a new one for the entire stick). Using all-apple tools should make that go smooth. 

Then just boot from the external stick by holding down opt at boot, and complete the guide. 

When in doubt or in need in help, always boot in verbose mode to get more information, and above all, do not over complicate things.