You dont need to disable secure boot if your trying to dual boot with Windows 8.1 as there is support for that with UEFI.
Next Windows 8.1 is a Preview build and does not have support for VHD usage. That type of support is only for the RTM Builds which is not available. You have to cleanly install Windows 8.1 preview in a pure dual boot environment.
Last but not least, you could never have it formatted to FAT32 as Windows no longer supports an install to that type of File System anymore. It will only install to NTFS. If you want GPT, that is your choice but I can guarantee you that if you have UEFI, that GPT is already on. You cant have UEFI with the disk setup as MBR. You only get the UEFI with GPT. So your already setup for that.