Any ideas how to revert the SD expansion?
From the forum thread discussing the topic: http://forum.arduino.cc/index.php?topic=236696.0
[quote author=Federico Fissore date=1398843712 link=msg=1702476]If you encounter any error and would like to restore your yun to its previous state, all you need to do is to reboot it with the SD card UNplugged, then edit file /etc/config/fstab removing the following section
option target '/overlay'
option device '/dev/sda2'
option fstype 'ext4'
option enabled '1'
option enabled_fsck '0'
option options 'rw,sync,noatime,nodiratime'
and reboot once more
I have tried to reset my Yun by doing a 30sec hold on the WLAN button, but this does not seem to have an effect as far as the partitioning goes
It's not clearly stated, but be aware that the 5 second and 30 second WLAN RST presses only work when the Yun is fully booted up - it's not like a lot of systems where you press and hold a reset button during power up. This often trips people up. Wait until the Yun is fully booted before you start to press the button. The reset process can take some time, so make sure you give it plenty of time to perform the reset operation and boot back up again before removing power or resetting it.
That being said, I don't know whether the 30 second reset really does revert the disk expansion.
I also cannot start the Arduino unless the SD card is inserted.
Then you may have an additional issue, as restarting without the card is the first step in reversing the disk expansion. I would suggest that you load the YunSerialTerminal example sketch, and start up the serial monitor. Then, tap the YUN RST button and let it (try to) boot up without the SD card. Finally, copy all of the boot messages from the serial monitor and post them here. It will help us figure out why it's not booting.
Have you tried to expand the Yún disk space again with another size?
One of the first things the expansion sketch does is check to see if the expansion is already set up. If so, the operation aborts. You can't re-run the operation without manually reverting the changes as described above.