cannot find external hard disk but connected
 

hi guys
i have been reading some postings. and many of them has problem similar like mine. The difference is i use HP Pavillion notebook. But have an external hard drive, which i want to share both with my iMac and HP notebook. My disk shows mac extented. Can anyone pl! help me how to to reformate the disk to MS-DOS.
i went with this problem to service center from where i bought it thinking that this the notebook problem. but they even dont know and telling me to check the whole system. and which ofcourse costs me 150 bucks
can anyone Pl! tell me how to reformat the disk.
i would really appreciate ur help or suggestions.
urgyen7