Reader Walt Pinkston has an impatient friend who was a little too anxious to put old data on a new Mac. He writes:
A friend dedcided to remove the internal hard drive from an old Mac, put the drive into an external enclosure, and hook that up to his new Mac. But now the new Mac doesn't recognize the drive and is asking him if he wants to reformat it. What's the best and safest way for him to move that data over to his new Mac?
With the idea of saving your relationship, let me start by saying that your friend's idea wasn't entirely boneheaded. There are indeed conditions under which you can jerk a hard drive out of an old Mac, shove it into an enclosure, and boot another Mac from it. Those conditions include having an operating system compatible with the new Mac and a drive formatted in a compatible way. In cases where the OS is quite old and the computer quite new, there can be problems as new Macs often demand an operating system no older than the one that shipped with them.
What I'd suggest in this case is to do things as Apple intended and use Migration Assistant. This will require your friend to put the old drive back where he found it (inside the old Mac), or connect the enclosure to the old Mac and boot from it.
When you perform a clean install of Yosemite (or first set up a new Mac) Migration Assistant appears in the form of a query about whether you want to transfer data from another Mac, Time Machine backup or startup disk, or Windows PC. If you skip this step, all is not lost as you can invoke Migration Assistant from the new Mac once it's up and running. You'll find it in /Applications/Utilities.
I needn't walk you through the complete process (because I've already done so here), but the gist is that you need to first establish a connection between the two computers. This can be a network, FireWire, or Thunderbolt connection. Launch Migration Assistant on each Mac and then choose to transfer the data from the old Mac to the new one. Select the source and destination volumes on each Mac, the data you'd like to transfer, and have at it. Depending on how much data is being transferred, this can take quite awhile.
The existence of the Mac App Store provides us with another option, and that's to start really fresh. If the bulk of the apps on the old Mac were purchased from the Mac App Store you could download copies on the new Mac and then simply transfer the data files over from the old Mac via the network. With IMAP email accounts that auto-populate your Inbox and personal data synced from iCloud, there may be less reason to copy most of the data from an old Mac to a new one.
Although this means a great deal more hand-holding on your part, it does let you freshly evaluate exactly what you do and don't want on your new Mac. I recently did something like this with a MacBook Air (that had limited storage) and I was surprised at how many apps I could do without.