I think I found my answer.
http://www.compactflash.org/faqs/faq.htm#dvsDual Voltage Support?
CompactFlash cards support both 3.3V and 5V operation and can be interchanged between 3.3V and 5V systems. This means that any CF card can operate at either voltage. Other small form factor flash cards may be available to operate at 3.3V or 5V, but any single card can operate at only one of the voltages
Can anyone confirm please ?
Thanks