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I've been searching for this forever... By default EFI partition on Mac is 200 MB. I have a special Windows 10 image (enterprise compilation) I would like to install on the Boot Camp partition, which requires that the EFI partition be 500 MB due to the image installer boot.wim file size, which is 290 MB.

The way I tried to do this is first install MS Windows 10 image downloaded from MS website by using Boot Camp Assistant to create and prepare Windows partition, then startup from the USB drive that includes the Windows 10 Enterprise image, deploy the image on top of the Windows partition, and reboot starting up from Windows to continue installation, but because I cannot copy boot.wim file onto the EFI partition, the deployment process cannot continue.

I've been trying different methods to change EFI partition size, using diskpart when starting up from the USB drive with the Windows image, or diskutil when starting up from macOS. None of which allowed me to resize the EFI partition:

  • On diskutil I get: "file system format does not support resizing".
  • On diskpart I get: "You may not extend OEM, ESP, or recovery partitions".

Is EFI partition resizing even possible?

Due to the fact that this is a relatively new Mac, I don't think I'll have any problem reformatting the drive for this to work, and reinstall Sierra from the Recovery Mode, if necessary (Of course if it's possible to change the partition size without losing data, that's the best solution).

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The ESP uses FAT32, so in theory, any tool that can resize FAT32 on a GUID Partition Table (GPT) disk can do the job; however, to do so, you'd need to shrink the partition that follows the ESP by changing its start point. This can be a relatively risky operation, so if at all possible, you should look for another solution. Also, some tools (like GParted in Linux) seem to be more likely to damage a FAT partition when resizing it than to damage other filesystem types.

Several other alternatives exist:

  • It's safer to shrink a partition by moving the end point than by moving the start point. Thus, you might consider shrinking some other partition from its end and create a new FAT partition in the freed space. This should give you space to hold the file(s) you need.
  • In theory, you should be able to store the boot.wim file on an HFS+ partition, since Apple's EFI can read HFS+ just as well as it can read FAT. This presumes, though, that boot.wim won't require write access, and that the file won't need to be read from Windows. Note that most recent OS X installations use encryption or an LVM setup, which the EFI can't read, so you'd need a plain unencrypted HFS+ partition.
  • At least two NTFS drivers are available. One comes with my rEFInd boot manager and the other comes with EFI drivers package. By using such a driver, you could theoretically put boot.wim on an NTFS volume. Some of the same caveats from an HFS+ location apply, but Windows would be able to read NTFS.
  • You could use another disk entirely -- say, a USB flash drive.
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  • Just to extand it a bit: The ESP (EFI system partition) may use any format it wants, but it must support at least the FAT32/EFI filesystem. It may also allow others; Apple has devices which use and boot from a Apple HFS formatted ESP.
    – Hennes
    Commented Nov 8, 2016 at 17:00
  • No, technically the ESP must be FAT (or, to be more precise, the filesystem that the EFI spec describes, which is basically identical to FAT). The spec mentions in one or two places that it should be FAT32, as opposed to FAT16 or FAT12, when used on a hard disk, but that requirement is pretty soft. An EFI firmware implementation may support additional filesystems (Apple's supports HFS+ and a few support ISO-9660), but using anything other than FAT on an ESP means it's technically not an ESP. It might work on some platforms, but it's not technically an ESP.
    – Rod Smith
    Commented Nov 8, 2016 at 23:04
  • @RodSmith thanks, I was hoping that I won't need to mess around with Macintosh HD partition starting sector, which follows the ESP, because it means that I have to reformat the drive from USB, remap the partitions sizes to match my needs, and reinstall macOS from recovery mode onto the newly moved partition. I've never done this before and I hope not to cause any issues preventing me from reinstalling macOS.
    – erutan
    Commented Nov 11, 2016 at 14:48
  • Regarding your suggestions: * I thought that boot.wim has to be on the ESP, so having a FAT partition after macOS won't do any good... * The boot.wim file is required to finish the deployment of Windows 10 so I can't put it on HFS+ partition. * Not sure I understand what you meant, but again, the file is needed on the ESP.
    – erutan
    Commented Nov 11, 2016 at 14:58
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    Thanks @Rod you helped me greatly! I was able to overcome this by creating another EFI (marking the old one as regular data), performed the deployment and reverted the drive state, and it worked!
    – erutan
    Commented Mar 16, 2017 at 15:13

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