While erasing partition in the drive got stopped with an error such as: Disc erase failed,couldn’t unmount disk or Disk Erase failed with the following error :
Normally,the couldn’t unmount Disk error is attributed to circumstances where the boot drive is being modified or is being used by an application or process. So the first thing to do is to startup the Mac from another bootable drive such as an external drive or OS X Recovery. You can then run Disk Utility from there. An external drive or a network drive is preferred if it is the internal hard drive you have an issue with, since the OS X Recovery is a partition on the same physical drive which may not be able to successfully unmount or modify your internal disk.