How to install Mac OS and Windows in external SSD

I use iMac with Mac OS and Windows in an external SSD. This is how to install these OS into an external SSD or HDD.

What I used.

• Windows running on PC or Mac
• iMac (21.5-inch, Late 2015)
• SSD: USB Caddy HDD Case
• USB Memory Stick 8GB*2

To begin with, prepare bootable usb drives for Windows and Mac OS.
Using Boot Camp Assistant in a Mac which you are supposed to install Windows in, select [Action > Download Windows Support Software] and down load Windows drivers for your Mac.

1. Start a Command Prompt as an Administrator
On Windows10, right-click on Start button and select the [Command Prompt(Admin)] option.
Enter [diskpart] to open diskpart.

Enter [list disk] to list the storages.

2. Select a storage

Select a storage which you wanna use. In my case, I entered [select disk 1].

Enter [list disk] again to be sure of the storage.

The storage you chose has [*] next to the storage.

3. Format the strage
Enter [clean] to erase all data on the selected storage.

Enter [convert gpt] to make GPT.

4. Make partitions and apply a Windows image

1. creat partition efi size=256
2. format fs=fat32 quick label=”EFI”
3. assign letter=X
4. create partition primary size=30000
5. format fs=ntfs quick label = “Windows”
6. assign letter=Y
7. list volume
8. exit

1~3. Make EFI partition on GPT. I assigned [X] to the partition.
*Do not use X, because this drive letter is reserved for Windows PE.*
4~6. Creat a partition for Windows. I assigned [Y].
7~8. List the volume and exit DISKPART.

5. Windows Image

1. dism /Get-WimInfo /WimFile:D:\sources\install.esd
2. dism/apply-image /imagefile:D:\sources\install.esd /index:2 /applydir:Y:\
3. bcdboot Y:\Windows /l ja-jp /s X: /f UEFI
4. exit

1. Check which index is your target. In my case, it is index:2.
2. Copy Windows Image[D] into partition[Y].
3. Copy BCD file in Y:\Windows into partition[X].
4.&nbsh;exit Command Prompt

6. Before Install

Open Disk Management. There is the Unallocated partition. Format it in exFAT. Connect the storage to Mac, and format the partition in OS X Extended (Journaled).
Place drivers for Windows that you downloaded via Boot Camp Assistant.

7. Install

Start up Mac, pressing and holding option key, and select EFI Boot to install Windows. You will start up Mac with option key, and select EFI Boot to use Windows after you install it. Install Mac OS from USB as well.