I have not used Time Machine before, but now need to back up to my external hard drive. How do I do this?

Some tips:

* Leave yourself plenty of time. Creating the first Time Machine backup can take several hours so don't start right before your appointment to have your HD replaced.

* Beginning to use TM is simple. Connect the external HD to your machne. Use Disk Utility (Applications>Utilities) to make sure it is formatted with the GUID Partition Table for the "Partition Map Scheme". Then open your System Preferences and launch the Time Machine control panel. TM will guide you from there. Essentially, you just let TM do its thing after enabling it.

* When your machine comes back with the new HD, you can use either Setup Assistant (which is available at first bootup following a HD replacement or an OS reinstall) or Migration Assistant (avaiable anytime in Utilities) to get all your data onto the new HD. It's best to use Setup Assistant to avoid various problems related to file permissions.

You could use Time Machine - just connect the external you want to use, launch Time Machine and it should ask you if you want to use it for back-up. Should be clear from there:

Have a look Pondini's mine of useful information regarding Time Machine:

http://pondini.org/OSX/Home.html

Another option is to use your external to make a bootable clone using SuperDuper! or CarbonCopyCloner:

http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

http://www.bombich.com/index.html

I would not use Time Machine for this task. Rather I suggest you clone to the external drive. You can then boot from the external drive to restore your clone. First, purchase Carbon Copy Cloner.

Clone using Carbon Copy Cloner

1. Open Carbon Copy Cloner.

2. Select the Source volume from the left side dropdown menu.

3. Select the Destination volume from the right side dropdown menu.

4. Click on the Clone button.

5. When completed select Disk Center from the Window menu.

6. Click on the Recovery HD tab then select the backup drive from the list.

7. Click on the button labeled "Create a Recovery partition for this volume."

Destination means the external backup drive. Source means the internal startup drive.

You now have a fully bootable copy of your iMac's hard drive. When you receive your iMac back after the drive replacement you can boot it from this backup and use Carbon Copy Cloner to restore the backup from the external drive to the internal drive. The process is identical except you will switch the Source and Destination volumes - Source will be the external drive; Destination will be the internal drive.

Because the newly installed drive will most likely have OS X installed you can skip Steps 5-7. You can now continue to use your external backup drive by setting up scheduled backups with CCC. No need to use Time Machine.

The last time I did a new backup to TimeMachine it took 11 hours, but it is free. Then after you get you Mac back, I would suggest using Migration Assistant. TimeMachine can be a time consumming task to restore from and as I just found out it can crash. The MA only took 2 hours, it did setup a new profile, but I got all my settings and documents back, including over 5000 songs in iTunes and 2000 pictures in iPhoto.

If you want to proceed that way, you should use Setup Assistant when first booting up - that avoids the duplicate account problems and the mess they may create. Personally, I will not use Migration Assistant again.

For the OP (reacher):

And, I agree with Kappy: I just had my drive replaced and cloning back from the external clone avoided all problems and was the quickest way to get back to where I had left off before taking the iMac in.

Don't forget to erase your internal drive after backing up and before taking in your iMac. You do not want to trust strangers with the information that is on your drive.