Here's a USB hard drive that has been split into 16 partitions, all of which will display separately on the Mac as separate volumes since they've been formatted, though they are all on the same drive. Update mac os. My new LaCie rugged USB-C drive showed only a capacity of 372 MB instead of the 4 TB that I had bought, when I formatted the disk on Mac OS X High Sierra. It turned out that I first had to create a GUID Partition Table (GPT) on it to use the full capacity. Find Out The Partition Table Type Select About This Mac from the Apple menu in the top-left corner, click on System Report. ![]() And select USB from the Hardware section. You can now see the different USB drives attached. Selecting one in the upper-right pane shows the relevant information: The drive displayed above is already converted. Before that it showed 374 MB capacity, and Master Boot Record (MBR) for the partition map type. You can see similar information from the: $ diskutil list /dev/disk0 (internal): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme 500.3 GB disk0 1: EFI EFI 314.6 MB disk0s1 2: Apple_APFS Container disk1 500.0 GB disk0s2. Look for the lines starting with 0. They show the partition type, either GUID_partion_scheme, APFS Container Scheme, or MBR_partition_scheme. Most USB drives, even those with only 2 TB of storage or less nowadays with a GPT but the 4 TB LaCie Samsung drives still used a Master Boot Record (MBR). Firing up disk Utility, right-clicking the drive and selecting Erase. Did not help. I had to us the command-line. Finding The Disk Identifier Before you can proceed, you must find out the disk number or identifier. The easiest way of doing this is to run diskutil list twice. ![]() Once with the drive plugged in, once plugged out (unmount first!), so that you can see it appear and vanish in the list. It can take a couple of seconds for a disk to appear and disappear. You should have now identified your drive as something like /dev/disk2 (external, physical). Verify and Repair Disk Permissions via Terminal (Mac OS X) Verify Permissions diskutil verifyPermissions / Repair Permissions diskutil repairPermissions / Verify and Repair Volume via Terminal (Mac OS X) Launch Terminal from your Dock, Utilities folder, or via an app launcher like Alfred. Once launched, type or paste in the following command. Diskutil verifyVolume [drive name] So, for your main hard drive, you could type or paste in something like: diskutil verifyVolume / Or, if you have an external drive attached, you can type or paste in the following: diskutil verifyvolume /Volumes/[drive name]/ So for an external drive named “BackupStuff” you would enter: diskutil verifyvolume /Volumes/BackupStuff/ If you see no messages, then you can assume that diskutil hasn’t found any errors, and you can move on. If, however, you get an error like “The volume Macintosh HD was found corrupt and needs to be repaired,” then you can repair the drive using diskutil as well. To repair the drive, you can issue the following command into Terminal: diskutil repairvolume / Or, for that external drive as above: diskutil repairvolume /Volumes/BackupStuff/. If unable to unmount drive on Mac You can try booting from OS X recovery by holding CMD + R at the time of startup. Once booted from OS X recovery, select Terminal from the utilities and then at the unix prompt enter: • diskutil list • then press return and look for the disk identifier and enter the following command with the disk identifier: • sudo diskutil unmountDisk /dev/(enter here disk identifier name) • Press return and enter your admin password if prompted. This should unmount all volumes of the physical drive.
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