I wanted to create a full copy of my MacBook Air in a disk image for archive. I can't save the disk from the booted system, so I restarted my Mac in Recovery mode (keys Command and R) on boot. From the recovery menu, I can launch Disk Utility, I select the 'Macintosh HD' drive, give the image a name, select the options "not compressed" and "read only". I select the destination disk and let the process begins... After a long time, just before ending the process, I have a message "error 28: No space left on Device".
My MBA has a 250 GB SSD disk, I have a 50 GB iCloud account, so in the case iCloud Drive is saved too (I don't think so...), there is just about maximum 300 GB to save. I tried to save on a Drobo (which claims being 64 TB in size, and have more than 2TB free), and on an empty 1TB Drive, and had the same result.