It took me a week, barking up the wrong three a lot, but I did it, an here is how: My premesis: A MacBook Pro 2017 with less than 10G space left An Ubuntu 20.20 with plenty of space An external 1TB SSD formatted with HFS+ and named "SSD" Start with your Mac. At first download Xcode from: https://developer.apple.com/download/more/?q=xcode directly to your external disk, changing the download location in the Safari preferences. I have chosen v12.0.1. Go to terminal an change directory to the mount point of your external drive. On your Mac now unpack the .xip file using: xar -xf Xcode_12.0.1 /Volumes/SSD/ This is going to give you a folder named "Xcode_12.0.1" with two files inside: Metadata and Content. Now switch to Ubuntu. Download the files from this repository: https://github.com/NiklasRosenstein/pbzx/blob/master/pbzx.c To compile it, I needed to download and install some old libraries from Ubuntu 14.10 on my Ubuntu 20.20. I can provide more Information about this if needed, but mainly it is about the following libraries and packages: libxar-1.6.1-alt3.x86_64 libxar-dev_0.7r1785-5ubuntu2_amd64 xar-1.6.1 libpng12-0_1.2.50-2+deb8u3_amd64 libwxgtk2.8-0_2.8.12.1+dfsg-2ubuntu2_amd64 libwxbase2.8-0_2.8.12.1+dfsg-2ubuntu2_amd64 I also had to look for the xar.h I found somewhere I don't remember. To make it easier I forked and uploaded the resources and a compiled version to: https://github.com/FolkertsHeiko/pbzx Sure you have to trust me, but you can. I promise ;-). If you want to do it yourself, here is the gcc command line, but do not expect it to work out of the box: gcc pbzx.c /usr/lib/libxar.so -lxar -llzma -o pbzx To mount the external volume you sure going to need the hfsprogs package: sudo apt-get install hfsprogs Next, mount or remount the HFS+ drive: sudo mount -t hfsplus -o force,rw /dev/sdXY /media/mntpoint or sudo mount -t hfsplus -o remount,force,rw /mount/point Now decompress the Content file with the self-compiled pbzx binary, both mentioned above, and unpack it with cpio, which already was installed on my Ubuntu System: pbzx -n /media/heiko/SSD/Xcode_12.0.1/Content | cpio -i This will give you another folder in your current working directory, named Xcode.app Go back to your Mac and link the Xcode-Application: cd /Applications sudo ln -s /Volumes/SSD/Xcode.app/ . Now run it with Finder, which is going to take a while for the first start. After that, Xcode should run from your external drive. I hope this is helpful. Happy coding, Heiko. Edit: Typos

Have a look here: https://developer.apple.com/forums/thread/110533?login=true&page=1#641207022

tl;dr: You are only going to need round about 45G on the external SSD. My solution is to unpack the .xip file containing Xcode manually there and run it.