GNOME Disks is built through Meson.
How to obtain and use it is documented here:
http://mesonbuild.com/Quick-guide.html

configure the build directory:
  meson builddir

(skip if not needed:
configure the build directory with options:
  meson -D libsystemd=true|false -D gsd_plugin=true|false builddir

list the available options and their current values:
  meson configure builddir

reconfigure a value:
  meson configure builddir -D libsystemd=true|false -D gsd_plugin=true|false
)

compile:
  cd builddir
  ninja

run GNOME Disks:
  builddir/src/disks/gnome-disks

to install to the configured prefix (defaults to system-wide /usr/local):
  cd builddir
  ninja install

produce a release tarball:
  cd builddir
  ninja dist



The above steps will build GNOME Disks against the system libraries
and the version of UDisks provided by the system.
This will fail if you cannot install the right dependencies.
Both JHBuild and BuildStream currently do not
have UDisks and libblockdev in their build manifests,
but it's possible to use Flatpak to build
GNOME Disks against UDisks and libblockdev
from git, and then start the built UDisks binary,
replacing the system daemon:

Using Flatpak to build and run GNOME Disks and UDisks (both from web sources):
  # (skip these two steps if you already installed the nightly GNOME runtime)
  flatpak --user remote-add --if-not-exists gnome-nightly https://sdk.gnome.org/gnome-nightly.flatpakrepo
  flatpak --user install gnome-nightly org.gnome.Platform//master org.gnome.Sdk//master
  # build GNOME Disks and all dependencies
  flatpak-builder app flatpak/org.gnome.DiskUtility.json
  # now run UDisks as system service, replacing your system's instance
  sudo bwrap --tmpfs / --proc /proc --bind /sys /sys --dev-bind /dev /dev --dir /tmp --ro-bind ~/.local/share/flatpak/runtime/org.gnome.Platform/x86_64/master/active/files /usr --bind /etc /etc --bind /var /var --bind /run /run --symlink usr/lib64 /lib64 --ro-bind app/files /app --chdir / --die-with-parent /app/libexec/udisks2/udisksd -r
  # or "sudo LD_LIBRARY_PATH=app/files/lib/ app/files/libexec/udisks2/udisksd -r" to run without any sandboxing
  # if you have host libs that are ABI-compatible with the GNOME nightly Flatpak runtime
  # (the benefit is that resizing/formatting binaries are available)
  #
  # in a new terminal in the current folder:
  flatpak-builder --run app flatpak/org.gnome.DiskUtility.json gnome-disks
  # or "LD_LIBRARY_PATH=app/files/lib/ app/files/bin/gnome-disks" to run without any sandboxing (see remark on UDisks)
You can also install the resulting Flatpak in your system:
  flatpak-builder --force-clean --user --install app flatpak/org.gnome.DiskUtility.json
  flatpak run org.gnome.DiskUtility
  # and uninstall it when it's not needed anymore:
  flatpak --user uninstall org.gnome.DiskUtility

This runs/installs GNOME Disks built against UDisks and libblockdev from git
but if you do not start the new udisksd with the bwrap trick,
the resulting Flatpak app will still use the UDisks version provided by the system. 
That is because Flatpak cannot run the built UDisks as new system service.

Note that the UDisks built with Flatpak is currently minimal
and does not include many UDisks features because the main
use case for Flatpak is currently running in the CI.
It's not meant for end users due to the fact that UDisks
cannot be bundled and people unaware of the version mismatch
will probably report strange bugs when running UDisks from
an old distribution.

Build a Flatpak app with your local source code changes:
  flatpak-builder --force-clean --stop-at=gnome-disk-utility app flatpak/org.gnome.DiskUtility.json gnome-disks
  flatpak build app meson --prefix=/app _build
  flatpak build app ninja -C _build install
  flatpak-builder --finish-only --repo=repo app flatpak/org.gnome.DiskUtility.json
  # Now you can already run it the same way as above (flatpak-builder --run app …, and restart UDisks through bwrap)
  # or you can continue to do the following step to create a Flatpak bundle.
  flatpak build-bundle repo org.gnome.DiskUtility.flatpak --runtime-repo=https://sdk.gnome.org/gnome-nightly.flatpakrepo org.gnome.DiskUtility
  # Install the resulting org.gnome.DiskUtility.flatpak file:
  flatpak --user install org.gnome.DiskUtility.flatpak
  # Finally, run the app:
  flatpak run org.gnome.DiskUtility
  # and uninstall it when you are done:
  flatpak --user uninstall org.gnome.DiskUtility

If you want to test things in your own development branches of UDisks or libblockdev,
you need to change the Flatpak manifest and point it to your repository/branch.
Here an example for the temporary change you need to do for libblockdev:
diff --git a/flatpak/org.gnome.DiskUtility.json b/flatpak/org.gnome.DiskUtility.json
index 2f89e2e3..16a20deb 100644
--- a/flatpak/org.gnome.DiskUtility.json
+++ b/flatpak/org.gnome.DiskUtility.json
@@ -189,7 +189,8 @@
             "sources" : [
                 {
                     "type" : "git",
-                    "url" : "https://github.com/storaged-project/libblockdev.git"
+                    "url" : "https://github.com/MYFORK/libblockdev.git",
+                    "branch" : "MYBRANCH"
                 }
             ]
         },
