Follow the advise from @ D.I. Johnson - First
If still having issues
This behaviour is fairly common when attempting to install macOS Ventura on an external drive with an Apple silicon Mac.
The key detail is that the external drive contains the Ventura System volume and Ventura Data volume, but the Data volume is essentially empty.
This usually indicates that the installer successfully created the APFS structure but did not complete the later stages of the installation process, which include configuring the external drive as a valid bootable macOS system.
One possible factor is the Mac’s startup security settings.
On Apple silicon Macs, Startup Security Utility is still available through macOS Recovery and should be checked, but the situation differs from Intel Macs with the T2 Security Chip.
The commonly referenced setting that allows booting from external or removable media primarily applies to Intel/T2 Macs.
On Apple silicon Macs, a correctly created and signed external macOS installation is normally permitted to boot, provided the startup security policy allows it.
To check the security settings, shut down the Mac, press and hold the power button until the startup options screen appears, select Options, and continue into macOS Recovery.
Open Startup Security Utility and confirm that the security policy is not preventing startup from the external installation. For most normal Ventura installations on Apple silicon Macs, Full Security should be sufficient.
Apple provides information about startup security settings here:
Use Safari extensions on your Mac - Apple Support
A more likely cause is that the Ventura installer did not complete the second-stage installation process. This can happen when the installer is launched from an existing macOS installation, such as Big Sur, and the external drive is selected as the destination. The installer may create the APFS System and Data volumes but fail to complete the final configuration steps required to make the drive independently bootable.
For installing macOS Ventura onto an external startup disk, it is generally more reliable to create a complete Ventura bootable installer and perform the installation by starting the Mac from that installer rather than launching the installer from the existing Big Sur system.
Apple provides instructions for creating a bootable macOS installer here:
Create a bootable installer for macOS - Apple Support
Before installing, erase the external drive using Disk Utility with the following settings:
Format: APFS
Scheme: GUID Partition Map
The installation should then be performed with the external drive selected as the destination.
Apple provides instructions for erasing and reformatting disks using Disk Utility here:
Use Disk Utility to erase a Mac with Apple silicon - Apple Support
The type of external drive may also affect the result.
If the 2 TB external drive is a mechanical hard disk connected by USB, it may not provide a reliable experience as a Ventura startup disk on an Apple silicon Mac.
An external SSD connected through USB-C, USB 3.1/3.2, or Thunderbolt is strongly recommended. A mechanical drive may allow the initial installation stage to complete but fail when the Mac attempts to restart into the newly installed system.
After the installation completes, manually select the external Ventura installation by shutting down the Mac, holding the power button until startup options appear, and selecting the external drive. If Ventura starts successfully, it can then be selected as the preferred startup disk.
Apple provides instructions for changing the startup disk here:
Mac startup key combinations - Apple Support
It is also important to ensure that the existing Big Sur installation is fully updated before attempting the installation again. The macOS installer may include firmware updates required for newer macOS versions, and an incomplete installation may prevent those updates from being applied correctly.
For Apple silicon Macs, additional information about using macOS Recovery and startup options is available here:
How to start up from macOS Recovery - Apple Support
Based on the symptoms described, the most likely causes are that the Ventura installation did not complete its second-stage setup, the external drive was not prepared correctly, or the external drive itself is not suitable as a macOS startup disk.
The nearly empty Ventura Data volume strongly suggests that the installation stopped before macOS completed its final configuration steps.
Startup Security Utility should be checked, but on an Apple silicon Mac it is less likely to be the primary cause than an incomplete installation, an improperly prepared disk, or an unsuitable external drive.
