External hdd not working properly

Hello, my mum has a maxtor 1tb external hdd which she does book work on i hvae used it on my laptop before and she uses it on hers now i connect it to my comp and it comes up as local disk (G:) not the maxtor device thing and i go to click it and it says do you want to format it ans i clicked no and i can't access it yet i can access it on my mums computer and my sisters comp and my sister external hdd does the same on my comp as well what do i do?????