You could do it from a bootable usb instead.
To make a bootable usb:
Format an 8 GB USB drive which should be called Untitled and formatted as Mac OS Extended (Journaled). The installer should be called Install OS X 10.11 Developer Beta.app and should be in your Applications folder.
Run this in terminal and wait about 20 minutes:
sudo /Applications/Install\ OS\ X \ 10.11\ Developer\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ 10.11\ Developer\ Beta.app --nointeraction
You can boot up from it by selecting it from the startup manager you get when starting your computer and holding down the option key.
If the installation creates a core storage logical volume you can revert it to get partitions back to normal by running these 2 commands in terminal.
diskutil cs list
diskutil coreStorage revert lvUUID
where lvUUID is the last lvUUID reported by the previous Terminal command.
Then restart for everything to get back to normal after you have run these commands in Terminal.
thanks for your answer, I'll give it a try and report back to you asap.