Hard to answer, not because of technical difficulty - but psychology. ;) So I will take the answer a bit easy...
A. The simple way
Make a Time Machine backup (if you do 1st times will get a full backup) so you will be able to restore anything that you created on the Mac. For this, you will need:
- one external HDD (big enough), connect it
- go to System Preferences
- use this external HDD as Backup disk for the Time Machine
B. As A. but with a bootable Time Machine HDD
For this, you will need:
- one external HDD (big enough)
- your OS X install DVD
- turn off Time Machine, if you have it on
- connect HDD
- insert install DVD
- start /Applications/Utilities/DiskUtility.app
- select your external HDD
- go to "Restore" Tab
- into "Source field", drag your Install DVD
- into "Destination", drag your external HDD
- triple check (so DON'T ERASE YOUR INTERNAL HDD!)
- press Restore and go to have a lunch
- after the restore, exit Disk Utility
- eject Install DVD
- go to System Preferences and enable Time machine
- select your external HDD as BackupDrive
- Make a full Time Machine backup
With the above, you get a BOOTABLE Time Machine HDD. When things go wrong, you can hold down Alt when your Mac is booting and select this HDD for BOOT and restore from it without needing the install DVD (faster restore).
C. Want even more?
Make working copies of your HDD. Like above, you will got a BOOTABLE external HDD, but not for install purpose, but for working from it. So, you can boot it and continue work as from your internal HDD.
For this you can:
- get a donationware: CarbonCopyCloner, or
- make a normal install into external HDD, boot it, and use the MigrationAssistant.app to migrate data from the internal HDD to freshly installed external HDD
Do you know that paranoia is the right state of the system administrator's mind?
Do B and C - So you will need two external HDD's. It is the best, because you can drop your backup HDD from the table, and after this crash-test, the drive will be probably unusable (broken). :) Anyway, for this solution, you need to not crash-test both drives.
With this, you will be able to WORK immediately (from the external drive) and restore from the Time Machine (from the second external HDD).
Know Murphy?
- Anything that can go wrong, will go wrong. (Hang up Murphy!)
- Your upgrade will fail (but usually it doesn't)
- Your backup will fail too (but usually you don't need it)
- so... take it easy ;)
Ps:
Here are more ways, e.g.: over the network from the command line with rsync command, etc. etc...
YOU REALLY WANT TO READ
It won't matter as far as the software goes since Lion runs the migration script one time only on your Snow Leopard data whether you migrate it in from a backup or it's there when you do the upgrade to Lion.
From a human standpoint, I would encourage you to make a "test" account (pick a different short name to prevent conflict or confusion when migrating your "real" account later)
Updating and testing Lion lets you build confidence the Lion is working on your repaired hardware.
- Run Snow Leopard updates to 10.6.6 at least to get the App Store if needed
- Install Lion
- Run all updates
- Test Lion in the test user. Once you're happy it works, migrate in your user data.
There's no right or wrong here - so do whatever makes the most sense to you. You've had a rough spell with the first upgrade / hardware failure - no need to over complicate things unless you really feel more steps is better.
Best Answer
Before you start, I would strongly recommend MacWorld's excellent series of articles on upgrading to Mavericks.
For taking a copy of your internal disk Carbon Copy Cloner is the best way to go. SuperDuper! is good too, but it's not compatible with Mavericks which might cause you problems if you want to back out. You can use CCC to create a separate bootable external disk (or even an internal partition). Try to use a USB 3 or FireWire disk as USB 2 is painfully slow to boot from.
Once you have taken your copy, upgrade your internal disk to Mavericks. If you don't like Mavericks you can undo your work by booting off your external disk and from there overwrite your internal disk using CCC.