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I have an iMac, its OS (EL Capitan) is located on an external hard drive. I want to install WIndows 10 as a second OS on this drive.

But, since it is an external drive, I can't devide it into several partitions, using BootCamp Assistant.

Is there any way I can do it? Maybe I should use different software? Even solution, that includes erasing of the data is acceptable at the moment!

Thank you in advance!

  • Since the question involving installing Windows on a external drive has already been asked and answered, I will ask what is the model/year of your Mac and how is the external drive connected? Is it USB 2, USB 3, Thunderbolt or other? – David Anderson Nov 5 '16 at 16:59
  • @DavidAnderson It is USB 3. The problem is that currently working Mac OS is installed on the same drive. But I also have an old Time Machine backup on another external drive, so maybe I should somehow use this backup during the process. Anyway, I would be very gratefull for an advice, and a detailed explanation of the process. – Rumata Nov 6 '16 at 20:25
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You can actually divide external drives into several partitions.

To do so you just need to add a partition table to the drive using Disk Utility. Unfortunately that will render existing data on the drive unreadable by conventional means.

So in essence you'll need to copy your data to a backup, add the partition table, format the macOS partition and copy back your data. Then you can format the Windows partition and install Windows.

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    If ElCap is installed onto an external drive, a partition table (GUID) is mandatory (= it's already exisitng > no need to create one). You can then resize the OS X main volume with diskutil resizeVolume ... and create a second volume for Windows. The main problem is, to get Windows booting from an external (USB-) drive. – klanomath Nov 5 '16 at 13:02
  • I got it working, but it has been several years, so it might not be as easy now. There's a guide here: bleeptobleep.blogspot.dk/2013/02/… – jksoegaard Nov 5 '16 at 15:27
  • @klanomath I would be very gratefull for a step by step explanation of the process if it is possible. About getting Windows booting from an external drive - I thought it would work if I hold OPTION (ALT) during computer startup, and I have to see the OS selection screen? At least it worked so when I used the internal hard drive, if I'm not mistaken. – Rumata Nov 6 '16 at 20:32
  • @jksoegaard Thank you very much, but as I understand it, this guide implies that I have the Mac OS installed on an internal hard drive. The problem is that in my case the Mac OS installed on the same external hard drive, and if I have to format it in the course of -the Mac OS will also be erased. – Rumata Nov 6 '16 at 20:35
  • @MaximVelichkin apple.stackexchange.com/questions/218105/… I don't know if the method works if the external drive has an additional macOS partition. – klanomath Nov 6 '16 at 20:40

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