Mac - Convert MBR to GPT - Error Message - NeedRestart

Hi,

I have MacOSX running Yosemite and I have installed the latest trial of HDM.

I am trying to convert my existing SSD from MBR to GPT to be able to install the new macOS Sierra without loosing data

I have a 480gb ssd with 23gb free (HFS+).

Everytime I try to do the conversion I get an error message saying "NeedRestart".

I have restarted the computer several times and I am still unable to convert MBR to GPT.

Any ideas?

Thanks.

It's recommended to perform such operations from separate environment, for example - Bootable Recovery Media.