I used to have a clone of an earlier macOS on a 120 GB external SSD for testing/troubleshooting purposes, but today I changed that. I used TechTool Pro to make a MacOS Basics bootable diagnostic drive. While setting up the drive I discovered extra apps could be added to it, so I added DiskWarrior to the list. This is a bare SSD with a SATA interface. I can use USB 2.0/3.0, Thunderbolt 1 or FireWire 800 adaptors to boot up various Macs with the drive. These two pics show the diagnostic apps available on the

I used to have a clone of an earlier macOS on a 120 GB external SSD for testing/troubleshooting purposes, but today I changed that. I used TechTool Pro to make a MacOS Basics bootable diagnostic drive. While setting up the drive I discovered extra apps could be added to it, so I added DiskWarrior to the list.
This is a bare SSD with a SATA interface. I can use USB 2.0/3.0, Thunderbolt 1 or FireWire 800 adaptors to boot up various Macs with the drive. These two pics show the diagnostic apps available on the drive.

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2017-01-25T07:32:38Z