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I have a Windows 10 ISO that is 5.3 GB. The Boot Camp disk image that Boot Camp Assistant downloads is 2.8 GB. Together these are more than 8 GB.

Boot Camp Assistant creates a partition named OSXRESERVED of 8 GB that (I think) serves as the the Windows installation medium.

Boot Camp Assistant first copies the contents of the Windows 10 ISO to the OSXRESERVED partition. Then it copies the contents of the Boot Camp disk image to the OSXRESERVED partition. It then does an unattended install of Windows 10 and the drivers required for my MacBook Pro.

(Please correct me if I'm wrong.)

The problem is that 8 GB is not enough space. So Boot Camp Assistant gives the following error:

The Windows support software could not be saved to the selected drive. There is not enough space available on the disk.

Is it possible to either:

  • A) resize the OSXRESERVED partition (taking some space from either the BOOTCAMP partition or the APFS container partition)
  • or B) tweak a 'OSXRESERVED partition size' setting in macOS that is used by Boot Camp Assistant and run it again?

Thank you!

Output from distil info OSXRESERVED:

Device Identifier:         disk0s3
Device Node:               /dev/disk0s3
Whole:                     No
Part of Whole:             disk0

Volume Name:               OSXRESERVED
Mounted:                   Yes
Mount Point:               /Volumes/OSXRESERVED

Partition Type:            Microsoft Basic Data
File System Personality:   ExFAT
Type (Bundle):             exfat
Name (User Visible):       ExFAT

OS Can Be Installed:       No
Media Type:                Generic
Protocol:                  PCI
SMART Status:              Verified
Volume UUID:               C6464CB1-7861-398E-93D4-06D988D784FC
Disk / Partition UUID:     A558AF49-0824-425E-B120-CFB50ADC2053
Partition Offset:          178210734080 Bytes (348067840 512-Byte-Device-Blocks)

Disk Size:                 8.0 GB (7998537728 Bytes) (exactly 15622144 512-Byte-Units)
Device Block Size:         512 Bytes

Volume Total Space:        8.0 GB (7996440576 Bytes) (exactly 15618048 512-Byte-Units)
Volume Used Space:         5.3 GB (5298814976 Bytes) (exactly 10349248 512-Byte-Units) (66.3%)
Volume Free Space:         2.7 GB (2697625600 Bytes) (exactly 5268800 512-Byte-Units) (33.7%)
Allocation Block Size:     32768 Bytes

Read-Only Media:           No
Read-Only Volume:          No

Device Location:           Internal
Removable Media:           Fixed

Solid State:               Yes
Hardware AES Support:      No
  • 1
    How did you get a 5.28 GB ISO? The last time I download the October 2018 ISO (1809) the size was 5.08 GB. Also, if you have a OSXRESERVED partition, then it would be extremely helpful to add the output from the command diskutil info OSXRESERVED to your question. – David Anderson Jan 6 at 18:18
  • It's the Windows 10 Business editions from December 2018. I guess it got a little bigger, and maybe Apple needs to adjust its settings to accommodate this. I will add the output you requested, you can see the volume used space is 5.3 GB. In the mean time I will go and see if the consumer editions ISO is smaller (I have MSDN, so I can download it as well). – Michiel van Oosterhout Jan 6 at 18:30
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Apple provides instructions for installing Windows 10. See Install Windows on your Mac with Boot Camp These instructions provide a link to the Microsoft Windows 10 download site.

The October 2018 ISO (1809) has one file that exceeds the maximum file size for a FAT32 formatted partition. This can cause problems when the Boot Camp Assistant instructs a user to insert a USB flash drive which is FAT32 formatted.

Your Mac model is newer so you should not have this problem, since the OSXRESERVED partition is formatted ExFAT. For other users with old Macs, one possible solution, if problems occur, would be to use the April 2018 ISO (1803).

Is it possible to either:

  • A) resize the OSXRESERVED partition (taking some space from either the BOOTCAMP partition or the APFS container partition?)

    It has been awhile since I ran the Boot Camp Assistant on a newer Mac. From what I remember, there was no way to stop the installation and customize the partitioning. You may be able customize by not using the Boot Camp Assistant. For the older Macs, the steps for installing were outlined in the question How to install Windows 10 into a 2013 iMac without using the Boot Camp Assistant, USB flash drive or third party tools? The answer is a bit outdated. I could update the answer if you post the output from diskutil list. I will admit that I am not sure if you would be better using the unattended install or not. In other words, I am not sure how exactly the OSXRESERVED partition is removed.

  • or B) tweak a 'OSXRESERVED partition size' setting in macOS that is used by Boot Camp Assistant and run it again?

    There is nothing the the Boot Camp instructions for changing this partition size. The command defaults read com.apple.bootcampassistant does not show any useful defaults that could be changed.

Another problem to look out for is the use of the Windows Recovery Environment partition. In some cases, the partition is not allocated enough space. This can result in an empty partition. The files that would have ended up in this partition are being placed in the Windows partition.

After Windows is installed, you can verify the location of the Windows RE files by entering the command given below in an Administrator Command Prompt window.

reagents /info
  • Thanks for the extended answer. I downloaded an older ISO (1803) that does not cause the problem with Boot Camp Assistant, and then I simply ran Windows Update after the installation completed. Thanks for adding the defaults read part, it confirmed B is not an option. – Michiel van Oosterhout Jan 7 at 14:53

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