Certain Apple Macs are more sensitive to power issues than others.
If you find that when the Saffire boots up, your mac turns itself off then this is because the mac sees 'too much' power being drawn and will turn off as a safety measure.
You will need to power the Saffire from the PSU to prevent this from happening. The Saffire will take the power from whichever device is supplying the most so you may also have to make sure that there is no power drawn from the mac.
You may have to buy a 6pin male to 4 pin female adapter then use a 4 pin to 6 pin cable.