Help! Migrating from one iMac to another

I have bought a new (used) iMac.
Machine 1 (Current): 1TB HDD, 250GB SSD (System and Apps) - both internal
Machine 2 (New): 1TB HDD (Internal), 250GB SSD (System and Apps, External, Samsung T5 USB3).

My questions are:

  1. Do I need to format the new external SSD prior to installing OSX (and Apps) on it?
  2. Will migration assistant give me the option to copy from an existing set up to the new one? Does it ask me separately where to put system and other files?
  3. Are the location of the user files set by what is the start up disk or do I need to set that separately?
  4. Does anyone have any other suggested workflows, tips or advice for this?


Could I just format the new drive as APFS, copy old SSD contents to the new one, and set it as start up disk? Then copy old HDD contents to new one?

Oh yeah, new SSD is external, so use something like Carbon Copy Cloner to make a bootable backup onto it from the old iMac’s SSD.
From there you could restore the 1T from a backup, or perhaps start the old iMac in target disk mode and copy stuff over.

I agree with @JohnAtl, use Carbon Copy Cloner to clone your existing iMac to the external hard drive. Then you can clone from the external to the hard drive to the new iMac. Carbon Copy Cloner offers a free 30 day trial. I highly recommend the software. It will ask you to format your external drive as part of the copy process and it will also clone the recovery partition onto the new drive.

Also, depending on where the new iMac came from, you might want to do a secure erase on the drives in case the previous owner had something nefarious on them.

It sounds like you are currently using an iMac with a Fusion drive setup and moving to an iMac with just a traditional HD (spinning disc) and plan to boot from an external SSD, is this correct?

  1. Yes, you will need to format the external SSD first.
  2. No, it does not give you that option. It is expecting you to put everything in one location.
  3. Yes they are.
  4. Advice offered so far is good. I would need more info before suggesting next steps. It sure sounds like you are trying to recreate a Fusion drive setup the hard way.

Thanks all, very helpful. I already use CCC for backups so all good. @MacGuyMI no it’s not currently a fusion - just an SSD installed to replace the optical drive.

It sounds like it’s not too daunting. But to help me get my head round things, can I ask what is the difference between:

  1. creating a bootable clone of my current SSD onto the new drive using CCC
  2. copying the contents of my current SSD (which I assume is already bootable) onto the new drive and setting it as the boot drive in preferences?

Will one work and the other not?


You could connect the external SSD to your old iMac and install an OS on it. Then you could use Migration Assistant to transfer the data from the internal SSD to the external SSD. After this completes, connect the external SSD to the new iMac and start it up holding down Option (use a wired keyboard for this). Select the external SSD and let it boot up. Once you are up and running, open Disk Utility and erase the the spinning HD (I am assuming it has an OS on it already). Now you can copy files over and link them to you User account. Use Thunderbolt cable, if possible, and Target Disk mode so you can simply drag and drop those files.

I hope this is clear and easy for you. I haven’t used CCC in years, but it is great software. I just can’t explain how it would be easier for your use case.

I guess it would depend on how you intend on doing the copy in your number 2 option. A “copy” is pretty much what is happening in option 1 with CCC. Migration Assistant as noted above would be another option. Your harddrive is a complicated beast with partitions, boot sectors, and file permissions. I think CCC (or a similar utility) is your best option to get all of your data off of your old Mac. Then on the new Mac you have the option to just clone backwards or install the OS fresh and run Migration Assistant to bring your data back.

@rlamarch my number 2 option is literally just to drag and drop copy the system files from the old SSD to the new external one (which I will boot from).

I guess maybe my question is just ‘is there something special about the way a bootable drive was created, or can it simply be a drive with the system files on which is chosen at boot up ‘?

At a minimum I think you would have to format the new disk and install OSX. After that you could manually copy your data files. I don’t think copying the system files over manually would be a good idea. As long as you have both iMacs, you could give it a try I guess. I still think your best option is to use CCC. If you are looking for a clean OSX install, do that to the new iMac and then use Migration Assistant to move your data and apps back to the new iMac form the external clone.

I’ll certainly use CCC but so I understand what is going on, I’m still not sure what is the difference between a CCC clone and copying the files over. What is CCC doing besides copying? Is it doing some hidden partitioning or something?

Yes, CCC is working at a much lower level than you as a logged in user. Your harddrive most likely has two partitions. One for recovery and one for your working OSX installation. You as a user cannot copy your recovery partition to a new disk. CCC can do this or the OSX installer can create this. Someone could be using bootcamp and have other partitions on their drive that can’t be copied. APFS complicates this even further with its containers. The operating system has tons of hidden files and directories that you won’t see. As part of the security and system protection at lot of those files have special permissions on them that you would not be able to duplicate on the target system as a regular user. Someone could have multiple user accounts on their Mac. By default, you are only going to see your account’s data. OSX uses symbolic links for files, so you could potentially copy the link but not the actual files where they are actually stored.

As I said earlier, for just your data, you could copy from one drive to another without issue. Even then some of that is a little tricky with things like email, photos, iTunes data, iMessages, etc. But system files are a different story.

My best recommendation would be to clone your existing harddrive to your external drive, do a fresh install of OSX on your new iMac, and then run migration assistant to move your user data and applications onto the new iMac.


That’s very helpful.