I wasn’t planning to do all my Blaugust posts on tech stuff, but it seems to be working out that way this week. 🤷 Today I learned about hybrid MBR/GUID partition tables, where the GPT (not that kind) on a disk is duplicated in an MBR table for compatibility purposes. Those reasons are probably outdated, unless you’re running a really old operating system, because GUID partitions have been around a very long time now. According to Wikipedia’s chart you have to go back to Windows 7 to find an OS that can access GPT disks but not boot from them, and Windows Server 2003 (fixed in SP1) to find one that doesn’t recognize them at all. Fun fact 1: This means a disk can have an MBR pointing to a partition that MBR doesn’t support, like a multi-terabyte filesystem! Fun fact 2: If Windows sees an MBR, it doesn’t bother with the GUID table, and it insists that partition is invalid! (Yes, even if you specifically formatted it with exFAT so Windows could read it.) macOS at least used to automatically create these when adding FAT filesystems, and if it still does, that probably explains how this external drive ended up with the hybrid MBR on it. Though if it does so when creating a >2TB system, I’d consider that a bug. Repair by Removing the Extra MBR Fortunately, it turned out to be easy to fix using the directions in this SuperUser post from 2012, because we don’t need the duplicate MBR for anything here in 2026. We used gdisk to create a new protective MBR table, leaving the GUID table unchanged. It fixed the problem and took less than a minute. We didn’t even have to jump through hoops to install it since we had a Fedora box handy and installing was just a matter of dnf install gdisk. Here’s what gdisk’s author suggested for converting the hybrid MBR disk to a standard GPT disk: Download GPT fdisk from its Sourceforge page and install it. (Versions are available for Linux, OS X, and Windows. I’ll assume you’ll do this from OS X.) Alternatively, you could run it from a Linux emergency disc like Parted Magic. [KV Note: If you have a Linux system already, check whether gdisk or gptdisk is available directly from your distro’s package manager.] Launch gdisk on your disk by typing sudo gdisk /dev/disk1 in a Terminal window. (Change the device identifier if it’s not as you presented earlier or if you use another OS for the job.) Type p to view the partition table to verify you’re working on the correct disk. If not, type q to quit without saving your changes and try again with another device. Type x to enter the experts’ menu. Type n to create a fresh protective MBR. Note that gdisk won’t confirm a change; it’ll just show you a new experts’ prompt. Type w to save your changes. You’ll be asked to confirm this action. Do so. With any luck this will fix the problem. If it doesn’t, though, you can use gdisk’s v option (on any menu) to have gdisk look for partition table problems. It can fix some minor problems automatically, but other times you’ll need to make explicit changes. See the GPT fdisk documentation on GPT repairs for details. I also looked up what a protective MBR is: On a regular GUID-partitioned disk, a protective MBR covers the whole disk to prevent older tools that don’t know about GPTs from clobbering the real partition table. ↑ It was really weird typing this up in 2026, because 99% of the time when someone is talking about “GPT” these days they mean ChatGPT (Generative pre-trained transformer) or something similar, not GUID Partition Table. This post by Kelson Vibber originally appeared on KV Tech Tips