Mac OS X Lion messed up my WD my book external hard drive

I have had my external hard drive for years and it works fine with my mac running on leopard operating system.  Yesterday, my brother plugged it into his new Mac laptop running on Lion and I can no longer access the external hard drive!  Help!! Everytime I plug it into a computer, it says, “the disk you inserted was not readable by this computer”.  I have the options of initialize, ignore, or eject.  How can I see my data again?  It doesn’t even work when I plug it back into the Mac OS X Lion.  What can I do? 

It sounds like something got corrupt between the 2 systems. I don’t know much about Macs so I don’t know if TestDisk will help or not. It has helped  a lot of people with problems on Windows. You might do some searching on that.