Sometimes users face problem when external Hard Drive won't mount on Mac. Initially it might be working fine but lately denies to mount. There can be various reasons for the same and one must rectify those issues at the earliest. If it is because of physical damages you can't do much about it except taking the services of technician. But if the drive is recognized and shows up in disk utility there might be some errors in the directory that creates problem where external hard drive won't mount. Further hard disk drive is a mandatory part of system which is mainly used to retain data. Its performance depends on many factors. If there is any irregularity then your external Hard Drive won't mount. Owing to its logical error or fault, mounting stops. Moreover, If you install more than one operating system, software conflict occurs and external Hard Drive won't mount. If experts are to be believed, it is not the physical damage but the logical error that causes mount failure. So as to retrieve the data lost when external Hard Drive won't mount, Mac Data Recovery Software should be used to retrieve data for future use.