Some time ago I wrote about 1803 upgrade problem I had with my desktop PC.
I had another upgrade problem with my Linx7 tablet. This tablet has only 1 gigabyte of memory and 32 gigabyte disk, but it can still run 1803.
In this case the tablet refused to update and rolled back simply because of too low free space in my disk drive C:\ (6 Gb). From the users perspective it would be great if Microsoft's update would first check if it has enough free space to complete the installation before even trying it...
First I emptied the recycle bin and used Disk Cleanup to get rid of extra files. That way I gained around 4 gigabytes.
Deleting Windows ESD installation files may free ~3Gb of disk space.
To figure out what else has been eating my disk space
I downloaded a tool called SpaceSniffer. Using it I figured out that Windows component store (c:\Windows\WinSxS) had grown to almost 7 gigabytes. There is a way to reduce it's size. You can use elevated command prompt (Start CMD as Administrator) by giving a command:
Dism.exe /online /Cleanup-Image /StartComponentCleanup /ResetBase
/ResetBase switch option deletes all superseded versions of every component in the component store. Note that you can't uninstall existing service packs and updates after completing this command, but it does not prevent installing future updates. I got more than a gigabyte freed by that trick.
If you still don't have enough free space, you can try compacting you os installation:
Compact.exe /CompactOS:always
Also you can gain a bit more space by removing you hibernation file:
powercfg /h off
With approximately 11 gigabytes of free space when upgrade 1803 was loaded, it finally installed without a problem.