MacOS – 2nd SSD as main boot disk, what do I do with old filesystem

You can definitely do it. But you either can't use your home folder to store your data for the most part, or you have to figure out how to do some kind of voodoo.

What I did is not supported by Apple, but it works just fine. And you can take it in for repair under warranty no problem. So there's nothing to stop you from doing it.

I got the information I needed here:

http://www.ransom-note-typography.com/index.php/SSD_and_Your_Home_Directory

http://www.tuaw.com/2009/05/14/tuaw-tip-moving-your-home-folder-to-another-disk-or-moving-it/

Basically the idea is you copy your user folder to a big conventional drive. Then you install Lion on a new SSD and create a user account with the same name. I named the big drive "Users" and then put the Home folders in its root.

Go to Users& Groups in System Preferences. Right click on the account. Choose advanced options. For the Home directory field, there's a "Choose" button on the right. Click it and navigate to the copy of the account on the new User drive, and select it. Reboot. You will now be using the user account on the conventional drive. Your Desktop, Pictures, Music, Downloads, Documents, etc. folders are now on the big drive, but it's just like it's on the boot drive.

A big benefit here is that you can format your boot drive and reconnect it to the User drive, and not touch your files at all. And you can clone the User drive and it won't contain your system files. (Or your Applications which remain on the boot drive.)

If it's not clear, ask questions and I'll try to clarify.

Besides the obvious issue of you having too much information to load onto the SSD, I've done this basic procedure in the past. However, it's much easier now using Time Machine, but you have a few options:

Time Machine

If you don't currently have a backup that can hold all your data, do yourself a favor and pick up a drive that can. Time Machine should be built into the OS you're currently running.

Do the backup. For 400 GB, you probably want to set it up before you go to sleep and let it run overnight. Once it's complete, unmount the drive and unplug it from the computer. Go through with the drive install and then install Lion. When the computer reboots, before you start to work with the setup assistant, plug in the new drive. At some point it will prompt you for a location from which to migrate data. Select your backup. Let it run for a while and when you get back, it will contain all the data you backed up, in a usable form. This is my current favorite method for cloning user workstations.

Super Duper

If you can't get an external backup, you can use SuperDuper to create a full-system backup from the active system disk. You'll have the ability to trim the data to the size of the disk to which you're copying in the interface. This route is a little more technical and requires a bit more time hands-on.

File Copy

I wouldn't recommend copying a user folder for a logged-in user due to open preferences files. You'll get all kinds of "File In Use" errors and it will probably exit the copy prematurely. Instead (if you're dead-set on using this method - which I advise against) create a second admin user and copy the folder over into a temporary location before staging it into the actual /Users/ location. You'll also need to make sure the permissions for the entire folder are set so that the user whose home you are copying can actually access and write to their files. To do this, you can use the Finder's Get Info on the folder and then add the user to have full permissions. Once it's staged into the /Users/ folder, you can log back in to the main user and delete the secondary admin. You may have permissions issues in the future using this method.

All that said, it's totally worth it to get an external to back up to for Time Machine to clone your new SSD from.