External devices can cause startup problems

As also documented in our tutorials "Identifying and neutralizing problematic internal/external devices" and "Startup fails; solving", problematic external devices (FireWire, USB, etc. -- particularly storage devices) can cause a variety of startup issues including inability to boot from the correct volume, kernel panics at startup and more.

MacFixIt reader Aadam Hassanali provides a case example:

"I had a very strange problem today. When attempting to restore a Mac Pro from the (Intel) DVD that it came with i had 2 different errors: When booting with an external (APM-formatted) disk still attached to the FireWire port, I got a grey no-entry symbol (as in circle with a backslash through it) and after 5 minutes still didn't boot. After disconnecting the external and booting from the DVD only, the system froze with the grey kernel panic screen. Luckily, after a third attempt (with the device disconnected), the system booted normally, allowing me to restore as normal."

Of course, the most obvious solution in the case of failed startup with external devices attached is to temporarily disconnect the devices then re-attempt booting. As in Aadam's case, however, minor problems may persist after the first restart with devices disconnected, necessitating a second restart.

If startup still fails, reset your system's NVRAM/PRAM via this process: Shut down your Mac, then start it back up while immediately holding the following keys: Command, Option, P and R. Hold the keys down until the computer restarts and you hear the startup sound for the third time. NVRAM and PRAM can sometimes harbor problematic data that lingers after disconnecting the offending device.

Clearing caches, removing specific kernel extensions and other workarounds listed in the aforementioned tutorials can also prove effective for device-caused startup failure.

Feedback? Late-breakers@macfixit.com.

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