Configurations with which we have seen some problems In the cases where USB was an alternative option, selecting the device as the startup disk works fine when connecting the same device to the Mac via USB. We have received a handful of reports from macOS Mojave users that attempting to select a Firewire-attached volume as the startup disk yields this same 'unable to bless' error. TRIM Enabler), undo that modification, then run the backup task again. If you used a third-party utility to modify macOS software (e.g.If the installation also fails, there is a compatibility issue between the enclosure and your Mac that makes it unsuitable as a startup device. Try installing macOS directly onto the cloned volume while your Mac is booted in Recovery mode.Try booting into Safe Boot mode (hold down the Option key (Intel Macs) or the Power button (Apple Silicon Macs) on startup, then hold down the Shift key as you select the backup volume as the startup disk).If your device is attached via FireWire and has a USB port as well, try attaching the device to your Mac via USB. As of macOS Catalina, Apple does not support booting a Mac via a FireWire-attached device.This can happen, for example, when using something like TRIM Enabler to modify macOS Storage drivers. We also see this more frequently on Yosemite when a critical kernel extension's code signature is invalid. We occasionally see this when trying to boot pre-2013 Macs from a USB 3.0 enclosure. Occasionally this also occurs due to a device driver conflict with the enclosure you are trying to boot from, or due to a firmware compatibility problem between the Mac and the enclosure. The most frequent cause for this is an attempt to boot your Mac from an incompatible (i.e. Related documentation If you see the universal 'No access' symbol after selecting your startup disk You may find the performance of one of these slower HDDs to be unusable for the purpose of booting your Mac from the backup. In particular, Apple's APFS filesystem performs relatively poorly on rotational HDD devices, and that performance is considerably worse for 5400RPM disks and disks that use Shingled Magnetic Recording. If your backup disk is a rotational HDD, then performance will vary from adequate to very poor, depending on the format of the backup volume, the operating system version, and specific performance characteristics of your backup disk. If your backup disk is an SSD, you can expect very good performance - comparable to the performance that you get when you boot your Mac from its internal SSD. The performance of your Mac while booted from the backup depends almost entirely on the performance of the hardware, and more specifically, the performance of the filesystem on that hardware. Bus powered enclosures often take a bit longer to spin up or simply don't make themselves available that early in the boot process. If the hard drive enclosure is bus powered, try plugging in its DC power supply before starting up your Mac.USB if you tried Thunderbolt, Thunderbolt if you already tried USB) and see if the volume appears. Repeat the steps above, but using another interface (e.g.Then start it up holding down the Option key, waiting another 30 seconds for the volume to appear. If the backup volume still does not appear as an option, shut down your Mac completely.If your backup volume appears, select it and proceed with the startup process. Detach, then reattach the backup volume's USB or Thunderbolt cable from/to your Mac and wait up to another 30 seconds.Press Option-Shift-Command-Period at the Startup Manager window to load any Option ROM firmware that is present and required for an external hard drive enclosure.If your backup volume appears at this step and the boot process proceeds past the Apple logo, skip to the section below. Wait about 30 seconds to see if the backup volume appears.Start up your Mac while holding down the Option key.
Attach the backup disk directly to a USB or Thunderbolt port on your Mac ( no hubs, no adapters, no monitor ports, no daisy chaining, no third-party USB cards).Detach all peripherals from your Mac except for the keyboard and mouse (including any secondary displays).Troubleshoot discoverability issues in the Mac's Startup Manager Also, for good measure, use Disk Utility's 'First Aid' utility to verify and repair any filesystem problems that may be present on the destination volume. If you are using an external hard drive enclosure or adapter, see whether your enclosure is listed at the bottom of this page as an enclosure that we've seen problems with in the past.
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