I have a pair of hard disks used by my NAS and one of them failed and I tried to use the other one as an external hard drive but I couldn’t format it on both Windows and Mac. On my mac it was not showing up in Disk Utility and on my windows 7, when I tried to format it, it failed. Finally, I found an article telling me to use Diskpart on windows. Here is how to do it:
1. Open cmd in admin mode.
2. Type in “diskpart”.
3. Type in “list disk”.
4. Find the disk name you want to format (in my case, mine is “disk 1”).
5. Select your disk by type in “select disk x” (in my case, it is disk 1).
6. Type in “clean”.
7. Open Disk Management and you will be prompt to initialize the disk.