A few days ago, after my 12-inch MacBook running the latest Sierra became sluggish, I restarted. After entering my account password, my Mac showed a white progress spinner that uses “spokes”—the kind of spinner you usually see at shutdown—would appear for several seconds, then disappear, replaced with a cursor that couldn’t be moved with the trackpad. This would recur over and over for hours.

I’d had a similar sort of problem crop up once during beta testing for software that I don’t want to mention the name of, as the issue was resolved for those of us on the cutting edge and the particular release of software is no longer in development. In that case, the spinner would appear and disappear, but no cursor, and much more rapidly. I contacted the former developer and followed his instructions to use Recovery and Terminal to remove all relevant files, but that didn’t solve this problem. Without the software installed, something else still triggered this slightly different behavior.

I went through a litany of options, including, after asking for advice on Twitter, ones I hadn’t considered:

• I could boot into Recovery (Command-R after startup or restart), so I booted into it and ran Disk Utility. The disk checked out fine.
• I booted into single-user mode and used the command-line tools to check for problems, and again, nothing emerged. After exiting into an interactive session, the same spinner issue cropped up. None of the errors I could find helped diagnose the problem.
• Safe mode didn’t bypass the point after login where the blockade happened, so it wasn’t of use.
• In Recovery, I reinstalled Sierra without erasing the disk. Same problem.

The one thing I didn’t remember to try, and wish I had, was attempting to screen share from another Mac—or even using SSH (remote secure Terminal-style access) to see if I could access the system while things were failing.

I began to wonder if it weren’t a video-card or GPU problem instead of a system problem. Apple and other computer makers engage a simpler mode in their graphics system at boot time, and then fire up the full power once the OS has initialized. Apple’s safe mode, for instance, disables accelerated graphics, for instance, which can cause video glitches.

To isolate whether it was hardware or the OS, I decided that since the disk was still reachable, I’d perform a full clone via Recovery using Disk Utility. Here’s how you do that:

1. Connect an external drive.
2. Boot into Recovery (Command-R).
3. Click Disk Utility in the list that appears and click Continue.
4. (Optional) If you have FileVault enabled, select your partition, and click Mount. When prompted, enter any administrative password.
5. Select the startup drive and then choose File > New Image > Image from “Partition Name.”
6. Choose the external drive as the destination.

This operation creates a mountable but not bootable clone as a disk image. Then I used Disk Utility to erase the drive. Unfortunately, this erased the Recovery partition, too! On restart, the Mac reverted to Internet Recovery, where it downloads the Recovery software and then installs and launches it. I used Disk Utility to restore my startup drive from the clone, figuring that might have cleared the condition.

IDG

(In Disk Utility, select the startup drive, click Restore, and then click Image to select the image. Some clones might not work in image form, but Disk Utility mounts them, and then you can select the mounted clone as the source for a restore.)