i've a mac powerbook g4 with mac os x 10.5.8 and a transcend 25m2 500 GB external hard disk . my mac used to show it on desktop as "time machine backups" but now it does not . have checked up disk utility also but can't see it . formatted it to fat32/exFAT but still not working . works well with a friend's windows 7 laptop . please help.