Last week Microsoft merged the Windows Insider fast ring with the skip ahead option and released build 18875 for both rings. This is the first 20H1 build for Windows Insiders on the fast ring and it seems many people have had problems getting the build to install.
On my machines the build downloaded and started installing but got stuck at the 7% point during the reboot phase, they seemed to eventually timeout and book back into Windows without installing the build. The error code is 0xca00a000 and hitting the retry button also brings up an error.
To fix it there are two things I did. First I deleted the contents of my %windir%\SoftwareDistribution\Download folder.
Next I stopped the Windows Search service which seems to be the cause of the update problem. In the search box type Services and launch the app, find the Windows Search service from the list and double click on it. Change the startup type to Disabled.
After I restarted the PC and then started the download off again the install then worked correctly. You don’t have to switch the search service back to automatic as the update process does this.