What your drive knows, and what it doesn't

Volume 13, Issue 6; 01 Mar 2010; last modified 08 Oct 2010

I recently had occasion to swap hard drives between two essentially identical laptops. A surprising number of apps knew the difference.

I have two essentially identical laptops, loaded 17” MacBook Pro's. One is my personal machine and one belongs to Mark Logic. I use my personal machine most of the time, but the fans have gotten insanely loud. I reported this as a warranty issue and got approval to take it in for service. (Yay!)

Not wanting to be without my laptop for several days, I decided to be clever and swap hard drives. The install videos at Other World Computing couldn't be more straightforward. (I had already bought the appropriate tools when I upgraded to a 500Gb drive.)

Following the swap, I found a curious mixture of systems and applications that could tell.

  • Not surprisingly, the iTunes authentication system could tell.

  • Also not surprising, Time Machine could tell. I suppose it's just possible that two folks could have absolutely identical but physically different hard drives. I really don't know how it could tell, though.

  • Some of the menu bar widgets were different. Apparently whatever makes the little flag icon is in flash memory somewhere and not configured from the hard drive?

  • The one that really surprised me was VMWare Fusion. It didn't like any of my virtual disks, it thought they were all “in use”. Stealing them back and telling VMWare I'd “moved” them was enough to recover, though, so no harm done.

My laptop is back and the drives have been switched again. I wish the problem had been fixed, but they claimed to be unable to reproduce it. (Boo!) I can tell you right now that it's still a problem.

I should push harder to get the fans replaced, the intermittent high pitched whine is definitely a Bad Thing. Of course, the 17” MacBook Pro is due for a refresh any day now, right?


The key for a lot of things (and for sure for Time Machine and iTunes) is the MAC address of the Ethernet port on the motherboard of the machine. If you have a problem that requires Apple to swap the motherboard, you'll have the same problems when you get your machine back.

("Amusingly", they suggest that you deauthorize your iTunes purchases before sending in your machine, which is difficult when the problem is serious enough to require a motherboard swap.)

—Posted by Claire on 02 Mar 2010 @ 05:09 UTC #

I had the same problem but managed to get the fan replaced, under warranty.

Watch for the CPU temperature, in my case the system was also overheating quite a bit.

—Posted by Pedro Morais on 02 Mar 2010 @ 09:37 UTC #

Macs cache a lot of information in the so-called PRAM. It is a common recommendation to "zap the PRAM" when you experience anything that seems weird. To do so, hold Cmd-Opt-P-R while booting.

About Time Machine: I think Time Machine identifies your backup by the Ethernet MAC, which is of course bound to your physical machine, not to the hard drive.

—Posted by Martin Probst on 02 Mar 2010 @ 09:49 UTC #