Q&A: MacFixIt Answers

How to address a corrupt hard drive before upgrading OS X, and other questions answered.


MacFixIt Answers is a feature in which we answer questions e-mailed in by our readers.

This week readers wrote in with questions regarding the necessity of TRIM on SSD devices when upgrading hard drives on Mac systems, reinstalling OS X 10.7 freshly on a hard drive instead of needing to upgrade or clone, and how to manage file-system corruption on a hard disk before upgrading to a new version of OS X. We welcome views from readers, so if you have any suggestions or alternative approaches to these problems, post them in the comments!

Question: The necessity of TRIM when upgrading to an SSD in OS X
MacFixIt reader Alexei asks:

"I have a mid-2010 MBP, running Snow Leopard. I'm planning to upgrade to Mountain Lion when it comes out, and also upgrade the mechanical HDD to an SSD. Typically, I would upgrade the OS, make sure things are stable, and then clone the drive to the SSD. However, I know that Snow Leopard does not support TRIM, while the newer versions do. I'm concerned that the OS won't know it's now running on an SSD, and will thus wear it out quickly.

Given these concerns, what would be the cleanest method of performing these two upgrades?"

TRIM support in OS X is, by default, on only for Apple-branded SSD drives. This is the case in Snow Leopard, in Lion, and likely in Mountain Lion as well. However, TRIM is not a necessary feature to have enabled for good SSD performance, especially with more modern drives that have alternative "garbage collection" routines. The OS will know it is running on an SSD, and the SSD will use its wear-leveling features to spread out use evenly to all of its transistors. As a result the SSD will not wear out any faster. The one benefit that TRIM offers is the ability to reset unused transistors, and therefore speed up the write process to them. In this sense a drive with TRIM enabled may perform better on some write processes than those without TRIM enabled, but will not wear out faster.

While Apple does not support TRIM on third-party SSD drives, there are some third-party tools (such as TRIM-enabler) that will enable it on a system, though these tools have no guarantees of success or benefit.

Question: Reinstalling OS X 10.7 freshly to a hard drive
MacFixIt reader Graham asks:

"There are lots of articles about reinstalling OS X 10.7 Lion on a hard drive, but all suggest that a previous system is cloned. My iMac died and I will buy a new hard disk, but this will be completely clean: how do I start afresh?

The iMac is not backed up (it is a spare and my MBP is). I also have a rescue flash drive with OS X and Disk Warrior and an extra external hard disk with OS X and utilities installed."

Apple sells standalone installer drives for OS X Lion at the Apple store; however, this is a relatively expensive option. Alternatively you can install Snow Leopard on your system if you previously purchased the install discs (or if they came with your Mac), and then upgrade to Lion through the Mac App Store, or use a secondary Mac such as your laptop to download Lion and burn it to a disc for use on your system.

If your rescue disk clones are for OS X Lion, then you can also use those to boot your iMac with the hard drive replacement, run the OS X installer, and have it download the Lion installation files to install freshly.

Question: Managing a corrupt disk before upgrading OS X
MacFixIt reader Roy asks:

"I already have a corrupt disk on my MacBook and I am currently running Snow Leopard. I do not have my original install disks. So will the install disks that I download and make a hard copy of when I purchase Lion also work to fix my disk now in Snow Leopard so that I can upgrade to Lion? In other words, how can I upgrade to Lion if my disk is already corrupt before I attempt the Lion install?"

Disk Utility cannot repair the boot volume. To do this, you will need to reboot to an alternative volume and run Disk Utility from there. In Snow Leopard or earlier versions of OS X you can boot to the OS X installation disc, choose your language, and then choose Disk Utility from the Utilities menu to run it and be able to repair the boot disk. Give this a try before you spend money on another utility program that you might ultimately not need.

If the drive cannot be fixed by Disk Utility, then I recommend you first address the problem before upgrading. In this case, your options are to use a third-party utility (like Drive Genius, TechTool Pro, or DiskWarrior) to try fixing the drive, repartition and format it to replace the current setup with a fresh one, or consider replacing the drive with a new one that is not showing these errors. Regardless of what you decide to do, be sure to fully back up your drive first (Time Machine can help you do this).

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