If you’re concerned about category 4 data and the SSD is going to a good home, simply re-formatting the SSD should suffice. As a result they can be much harder to sanitise properly than a hard disk. Unfortunately, SSDs perform a lot more low-level management with routines such as TRIM and wear-levelling. There isn’t any straightforward way of addressing that, but the risk is exceedingly low, even if that disk were to be passed to the best of data recovery specialists and cost were no object. Most secure erase features like that in Disk Utility can’t clean some storage blocks which have been marked as bad, so they might leave a small amount of data which could potentially be recovered. In theory, you might then wish to choose a higher level of security at this stage, but that will take much longer and you should ask yourself whether that is really worth it. If your hard disk falls into the hands of a state-sponsored intelligence agency or another well-funded group, they may still be able to recover some of the data which were on your hard disk. This is ample to ensure that someone can’t use a readily available tool to recover your old deleted files.
The most basic overwrites all free space twice: the first time it writes random data, and the second time zero bytes. Those three offer different numbers of passes in which the erase is performed. This is easily done using the secure erase option in Disk Utility – click on the Security Options button.ĭisk Utility offers three different levels of secure erase, above the Fastest setting which doesn’t overwrite free space at all. What you therefore need to do is overwrite all the free space remaining after the reformat. The reason that simple reformatting doesn’t do a good enough job is that the free space contains all your old files, and disk recovery utilities are designed to reassemble the contents of that free space into recovered files. In all other circumstances, you should take active steps to ensure that your data can’t readily be recovered from the storage. That is probably sufficient if your Mac is going to a close friend or relative who isn’t going to sell it (or have it stolen) in just a few days, and the private data are in category 4.
If you just reformat the hard disk, this makes it relatively easy for a third party to recover all or most of your sensitive data using a cheap and readily-available tool. The next question is what type of medium they are stored on: a traditional rotating hard disk, an SSD, or both in a Fusion Drive? Here I’m going to concentrate on the latter two, as those are the ones for which you make all the decisions. However, we’re prone to pretend that it really isn’t that important, until something goes wrong. The first two may seem irrelevant to almost everyone, but a lot of business information falls into the second category. Again, you need to satisfy yourself that it is destroyed. This should, for example, include all your keychains and any other account information. All other private data which you don’t want others to see.
You need to be confident that these are completely destroyed, but can choose how to do that yourself.
Don’t go it alone, but get proper security advice. If you ever handle any of these, you will be subject to national legal requirements and must follow those.
The most common reason for wanting to clean up sensitive files on internal or external storage is when you’re going to sell or give your Mac (or drive) away.