diff --git a/README.org b/README.org
index 54fc97d..1f80646 100644
--- a/README.org
+++ b/README.org
@@ -97,18 +97,26 @@ For some hardware the official Guix installation image won't do
 (e.g. unsupported wifi).  You can generate an installation image running the
 nonfree Linux kernel and nonfree firmware with the following command:
 
+#+begin_src sh
+guix system image --image-type=iso9660 /path/to/this/channel/nongnu/system/install.scm
+#+end_src
+
+Like the official Guix installation image, this will produce a read-only image
+with any changes made stored in memory.  As indicated below, you will need to
+run ~guix pull~ to download the Nonguix package descriptions, so will need
+enough memory to hold the cached channel code which can be several hundred
+megabytes.  As an alternative, you can create a writable image with the
+following command:
+
 #+begin_src sh
 guix system image --image-size=7.2GiB /path/to/this/channel/nongnu/system/install.scm
 #+end_src
 
-As indicated bellow, you will need to run ~guix pull~ to download the
-Nonguix package descriptions. Some free space on your USB thumbdrive is
-required for this operation to succeed. The ~--image-size~ option allows
-you to specify the size of the image and, as such, to allocate free space
-to it. The given value is purely indicative. It obviously depends on your
-thumbdrive capacity.
+The ~--image-size~ option allows you to specify the size of the image and, as
+such, to allocate free space to it.  The given value is purely indicative. It
+obviously depends on your thumbdrive capacity.
 
-Then you can write the generated disk image to a USB thumbdrive with:
+Either type of image can be written to a USB thumbdrive with:
 
 #+BEGIN_SRC sh
 # NOTE: This example assumes your thumbdrive is recognized by Linux as /dev/sdb.