MacOS – How much space do I need to make a bootable Catalina volume on the Mojave Mac

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.

Okay so I went ahead with the update anyway and it turns out that Migration Assistant is clever enough to recognise relocated user accounts.

In my particular case it recognised them, but provided no option to restore their contents (just the account itself), possibly because my users volume hadn't been recreated at this point.

However, after restoration was complete, and I'd recreated my users volume, all I needed to do was pop into Time Machine and restore the user folders from there, with the added advantage of the system still being usable throughout via my admin account.

For anyone in a similar situation, here are the basic steps that I followed:

1. Reinstall OS X as normal, at the restoring content stage select to restore from Time Machine, and restore as much as little as you like. Relocated user accounts at this point show up as 19kb with a note that they were relocated; select them to restore the accounts. You'll need at least one account on the system volume to login, so if you don't have any to restore then I recommend creating an administrator account; this has the added benefit that you can downgrade your other users to standard for security.
2. Disable Time Machine in System Preferences to prevent a premature backup.
3. Once you're happy the basic OS is set-up (updates applied etc.) you can now recreate the user-accounts volume. Make sure that Time Machine is disabled throughout.
4. Now locate your computer's old backup folder; do this by opening your backup volume in the Finder and opening Backups.backupdb, and it'll be one of the folders inside (usually just one for a single-machine backup drive).
5. If Time Machine didn't ask to inherit your backup drive, you'll want to make sure you inherit it manually (or it might see all files as new). To do this, open Terminal and enter sudo tmutil inheritbackup now take your machine's old backup folder and drag it onto the Terminal window to add its path. Your command should look like: sudo tmutil inheritbackup /Volumes/Backup/Backups.backupdb/Haravikk\'s\ Mac, you'll be asked for your password to run the command (must be run by an administrator).
6. Next you'll want to make sure you've associated the new volumes with their backed up counterparts (since newly created volumes will have a new unique ID). To do this open your machine's backup folder, and inside it open up the folder named "Latest", here you should see an entry for each volume that Time Machine was backing up. In Terminal, enter the following command: sudo tmutil associatedisk and drag the appropriate folder onto the Terminal window, then drag the corresponding volume on as well, your command will look like: sudo tmutil associatedisk /Volumes/Backup/Backups.backupdb/Haravikk\'s\ Mac/Latest/Users /Volumes/Users, hit enter to run the command.
7. Now when you open up your Users volume in the Finder, you can use Enter Time Machine from the Time Machine menu and use it to go back to see your old user folders, simply select each one you want and click Restore. You can do this several time if (like me) some user accounts are bigger than others.

Once you've restored all the user accounts, you should be safe to re-enable Time Machine backups, and the accounts should now work just as they did before (though some passwords may need to be re-entered).