Hi there, I solved the problem partitioning the other hard drive (not the SSD) and copying using dd the syslinux partition on the usb stick to the partition I create on the other internal HD.
This procedure is good if you have removed the optical drive and installed a SSD in the optibay.
First do all the above step to create a bootable USB stick with rEFIt and mount it
then using disk utility partition the "other" hard drive with the less dimension as possible. Please note that OSx will create a huge partition (in my case 70Gb) google to search a procedure to do this if you don't know how to do it. Give the partition the name you want for example FIRMWARE
Then in terminal:
find the IDENTIFIER of both the new partition in the internal hard drive and the partition SYSLINUX in the USB stick for example in my case the partition was disk0s4 and the usb stick SYSLINUX partition was disk2s2
so in the terminal
diskutil unmount /Volumes/SYSLINUX
diskutil unmount /Volumes/FIRMWARE
sudo dd if=/dev/disk2s2 of=/dev/disk0s4 bs=1m
Reboot your Mac and hold the alt key when it starts. Choose rEFit from the list of startup disks.
In the rEFit menu choose Boot Legacy OS from SYSLINUX note that you will have 2 of this options one with a ORANGE hard drive symbol (is the partition on the USB stick) and another with the GRAY hard drive symbol, choose the second one (the grey hard drive)
In the UNetbootin menu choose SeaFlash. You now boot into the firmware upgrade utility. Follow the firmware upgrade instructions/manual from now on.
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