I did some planning to ensure that the total size of a content database do not exceed 100GB by summing all the quotas of the site collections. i.e. More content databases were created.
There are a lot of blogs out there to move site collections and I am not going to duplicate that. Just a little something I learned along the way.
Make sure there is enough disk space for the move. In other words ensure the drive the log file for the originating content database and the target content database has enough space. If the site collection is 20GB make sure you have 25GB disk space each for the LOG's and another 25GB for the target content database.
If you don't have enough disk space the process will fail. This is what happened to me. But, I then made sure there is adequate disk space for the move. And then the process failed because the Site collection already exists in the target content database. I never cleaned up the previous attempt's writing to the content database.
I then made matters worse by using stsadm -o deletesite -siteid
So now the original content database had the orphan site collection. Luckily this can be fixed easily enough.
stsadm -o deletecontentdb -databasename
stsadm -o addcontentdb -databasename
The addcontentdb command forces SharePoint to remap all the site collections in the content database. So the data was never deleted, it was just "lost", or an orphan.