If you just did a simple, quick erase on a hard drive ten years ago, any competent data recovery software would turn up a goldmine of data.
Erasing a disk the quick way in those days only put a new filesystem header on the front of the disk, like replacing the table of contents of a book with an empty one, but leaving the rest of the pages in the book intact. They did this for speed; overwriting all the data on a disk takes many hours. That key is kept on the drive, and all data written to it is encrypted with that key.
When a secure erase is requested, that key is destroyed, leaving all the encrypted data unreadable. I decided to look into whether a Disk Utility erase does leave easy-to-read breadcrumbs behind, or whether it cleans up after itself. A disk—any disk—is basically a giant file, the size of the entire disk. The easiest way to look for data to be recovered on a disused disk is to scan it, beginning to end, and look for patterns that indicate useful data.
The first thing I needed to do to test this out was fill a disk with data I could easily find again.
To do this, I took the Ann Arbor office loaner MacBook—recently erased from its last borrower—and half-filled its disk with a bunch of files. Once in, the incantation to scan the disk—this will read the entire disk in 1 megabyte chunks, and pass it through a hex dump tool that we can use to visually inspect the data:. And a large portion of the output—which I stopped, because it would take far too long to visually read the whole disk—looked like this:.
All You Need to Know to use Disk Utility on your Mac
My next step was to reboot into the macOS recovery partition and erase the disk with Disk Utility like Apple advises. Opting to skip the install step entirely gave me the greatest chance to find any trace of the data. Once erased, I rebooted into Ubuntu one more time, and ran the same command.
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The output was much shorter this time—I let it run to the end, seeing no trace of my data, but just this:. So what does this mean? It means that whatever data you had before the erase is inaccessible to just about anyone who acquires your computer, which is great news for anyone who might want to grab a copy of Disk Drill and start digging. You might see a black screen, you could get an error message, or your computer may hang on the white Apple logo.
If your machine is running fine, this troubleshooting guide is not for you.
How to fix common Mac problems
You can, however, run First Aid on your drive anytime in macOS. Click on First Aid followed by Run to scan the disk for problems and attempt to fix them. By attempting to boot into Safe mode, macOS will scan and repair any errors it finds on the disk. Safe mode boots your operating system with only the bare minimum it needs to run. No third party kernel extensions will boot, login items are skipped, and it empties certain caches. To boot into Safe mode: Turn your computer off, then press and hold the Shift key while your computer boots.
If your computer has hung on boot, shut it down by holding the power button until the machine switches off around 10 seconds first. Should your computer boot properly, try restarting and booting as normal. You can then restart into Safe mode, remove the problem extension, and try again. Every Mac ships with Recovery mode, which lives on a separate partition on the startup disk.
Later you also may want to restore it. Here's how to do both. Keep in mind this requires an internet connection and will take a lot longer, since macOS needs to download the image first. Select it from the list on the left, then click First Aid followed by Run. File system consistency check or fsck for short is a longstanding Unix tool for checking and fixing drive problems.
Depending on the size of your volume, the type of drive you have installed, and the potential problem, fsck can take a while to complete. Patience and persistence is important if you want to fix your drive this way. Gonna be a bad weekend, as this is not what a Mac is supposed to look like during boot. This boot mode allows you to make changes to shared user resources. You can type fsck -fy to run fsck.
If you have a firmware password set on your Mac, Single User mode is not available to you. If you interrupt the process while it is making changes to your drive, you may incur data loss. Read More. It's called Apple Service Diagnostic, and we'll show you how to get it and use it for best results. Read More to learn as much information about the problem as you can. Specify your external drive and start the process.