I cannot read a external hard disk in mac computer!

I am using a 60G portable Samsung harddisk for working with my present acer laptop and some mac computers A few weeks ago. It works perfectly well whenever i plugged in my portable harddisk into the mac computers. They couol detect the harddisk and i could work with it.

but suddenly, i don't know why these days, all the apple computers(mac) are no longer able to detect the portable harddisk. (but at the meanwhile, it still works well with my acer laptop). I've tried many mac computers but still fail to work.

could you please kindly answer my question? thank you.