If your Catalina installation is stucked due insufficent space („no enough disk space“), and you can’t restart because your start volume is gone, here is the solution.
I had the same problem:
During the installation process the update to Catalina stucked due not enough disk space available.
Restart with previous Mojave OS was not possible because I could not select any startup volume – (it was not there to select).
If I did a restart (without selecting any start volume) anyway, it tried to install Catalina again and stopped immediately with the error message „no enough disk space“.
I tried recovery mode, disk repair (no errors found). Nothing helped.
No TimeMachine backup was available.
I tried also to reinstall mojave via recovery mode. Did not work either due „not enough space“.
Tried to clone the macintosh HD to an external HD: Did not work due OSS Error.
What else could I do? After a Call with Apple Support they told me the solution, that solves the issue.
- I installed Mojave on an external drive and booted from this external drive.
- Running the Mac again, I saved my files from the macintosh HD (internal HD) to the external device.
- After that I deleted the macintosh HD (internal HD) completly and installed Mojave again.
- After this I updated to Catalina and copied back my saved files from the external HD.
Hope this work around helps others with the same issue.
Let me know in the comments if this helped you. If you have any questions, just ask in the comments.