If you are a power user for both Mac and Windows base PC, you maybe tempted to use previously running Windows base system disk for your Mac system disk saving some cost.  But, please be aware, Mac GUI Disk Utility cannot erase (wipe clean as raw disk) all the Windows formatted disk.

Typically, this happens with previous MBR (Master Boot Record) Windows system disk.

Mac can erase disk with Mac OS Extended (Journaled) format, however, GUID Partition Table Scheme (Windows system refers this disk format as GPT disk) will not be added on the disk. GUID Partition Table Scheme is a minimum requirement for macOS to be installed.

Furthermore, Mac computer Disk Utility cannot make the disk as raw disk (not partitioned not formatted disk).

Best way to deal with this is to use Windows base utility program before make it Mac operating system disk.

I use IM-Magic Partition Resizer which is a small commercial utility for Windows computer before move it to Mac computer.

To make sure it will work with Mac computer, I make the Windows base disk as raw (not partitioned and not formatted) disk.

Simply delete all the partitions and make it raw disk.

Then when you use it with Mac computer, Mac will give you an option to make it Mac OS Extended (Journaled) format with GUID Partition Table Scheme.

As replacing internal Mac computer hard disk is not so easy, this will ensure the disk will work as Mac operating system disk.

If you require any Help for dealing with macOS installation (Sydney CBD, Sydney North Shore – particularly in Chatswood, Roseville, Lindfield, Killara, Gordon, Pymble, Turramurra, Wahroonga, Warrawee, Hornsby, Macquarie Park and St Ives area) please contact Harry 0417 424 909.  Or refer to our Service page.

01 Previous Windows 7 system disk (MBR format) erased by Mac Disk Utility (When you erase the disk, it does not give an option with GUID Partition Map Scheme)

Previous Windows 7 System Disk Erased under Mac

2. But you cannot install macOS on the disk

Can not install macOS on the disk

3. Remove the disk from Mac and connect it to PC again – shown as Unknown-disk (Disk remains as MBR disk)

Shown as Unknown-Disk

4. Delete partition

Delete Partition

5. Confirm deletion

Confirm Deleting Partition

6. Disk becomes Unallocated (Raw) disk

Unallocated Partiton

7. Apply change

Apply Change

8. Complete


9. Connect it again to Mac

10. Erase the disk (It will give you GUID Partition Map Scheme option)

11. Erase complete

Erase complete

12. Now you can install macOS

In a nut shell, best to use raw disk when you install macOS.

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