Previous research suggests that I need an LVM. However, I can't find anything for what to do about data being on both drives. I took the hard drive of an existing, running 10.04 computer and put it into another existing, running, machine. I want to be able to access the files like they were on the local(booting) hard drive. For example, having the applications that were previously installed on the "foreign" drive accessible through the "Applications" button. Any suggestions?
You could merge the contents using a union mount, but this would not do what you want. All the system files from the new hard drive would mask the entire system on the boot hard disk, so you would effectively be trying to replace the whole operating system while it is running, not just the newly installed applications.
If you want to make the second system into a "clone" of the first, with all the same applications installed, you can use aptitude. First, you run aptitude-create-state-bundle on the first system. This creates a record of all the applications you have installed. You copy this file to the second system, and then use aptitude-run-state-bundle on it. This will install all those same applications on the second system.
It's not possible because: 1) Changing from non LVM to LVM requires to rebuild your partitions (data loss) 2) You can not merge existing running systems, it would bring all the sort of problems like only keeping the package database from one system You only option is to backup user data, reconfigure the diks with LVM and reinstall.