Months ago I performed a clean install of Yosemite, and as you can read elsewhere, drive was formatted as Core Storage. Later on I partitioned and put on Windows 8.1 through Bootcamp.
Worked well for months (still working actually). I was booting holding the option key and I was able to choose between the OS X and Windows partitions.
However, Bootcamp does not provide drivers to read Core Storage disks. So, in order to get read access to the OS X drive from Windows, I followed the most common suggestion I could find:
diskutil coreStorage revert <UUID>. It is well (though unofficially) documented and worked with me also - OS X drive is HFS+ again and I can read it when booted into Windows.
The only issue I could find is that now, when booting holding option, I am presented a third option called EFI boot. I am not brave enough to click on it, but would like to know what is it, why it's there and eventually how I can get rid of it. It is not shown in Disk Utility of course.
Here is an image pretty similar to my case:
diskutil coreStorage revert I had just Yosemite and Windows options; now there's this new guy. However this happens only in the boot screen; there's no EFI boot in Disk Utility nor in the Startup Disk view.
/dev/disk0 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *750.2 GB disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_HFS Macintosh HD 609.3 GB disk0s2 3: Apple_Boot Recovery HD 650.0 MB disk0s3 4: Microsoft Basic Data Windows HD 140.0 GB disk0s4 /dev/disk1 #: TYPE NAME SIZE IDENTIFIER 0: CD_partition_scheme *116.8 MB disk1 1: Apple_partition_scheme 101.7 MB disk1s0 2: Apple_partition_map 17.9 KB disk1s0s1 3: Apple_HFS Vodafone Mobile Broa... 31.9 MB disk1s0s2 //last one should be my internet key actually plugged in.