You can’t boot OS X from a USB hard drive.
That might be old news to the more tech savvy among you. But it’s something that a lot of people don’t seem to know. In fact, only one person I asked—including many folks working for prominent drive manufacturers—knew anything about this limitation in OS X.
Here’s how I found that out while working on the test suite for our
external hard drive reviews. Along with our standard set of real-world speed tests, we decided to make sure that each external drive we reviewed could be used as a bootable backup of a user’s system.
Using Carbon Copy Cloner from
Bombich Software, I created a disk image of a system running OS X 10.3.8. Then I’d mount the disk image, set it as the source and then set the external drive as the target and hit the Clone button.
The first couple of drives I tested were FireWire-only and I had no problem booting from these newly cloned external drives.
The next drive had only USB ports. I attached the drive and was able to set it as the target and Carbon Copy Cloner went about its business. When the clone operation was finished, I went to System Preferences/Startup Disk. I found the USB drive in the list of available startup systems, selected it, and restarted my Power Mac G5. The computer mulled it over for a minute, but decided to boot up from the internal drive instead. I tried again, but had no luck.
I looked online and found one drive manufacturer and a couple of forum postings directing people to Apple Knowledge Base articles stating that a “device such as a SuperDisk, Zip disk, or other USB storage drive can be used to hold a valid system folder and used at startup.”
It wasn’t working for me, so I asked a few people if they’d ever tried it. The general response was, “Boot from USB? I haven’t tried it, but I don’t see why not.”
Assuming that I had done something wrong, I decided to try installing the OS directly from an OS X 10.3x disk. After inserting the disk and rebooting, I finally got the answer. The installer recognized the drive, but would not let me install onto the USB drive saying that OS X cannot startup from this device.
I called the drive’s manufacturer to see if they were aware of this problem. It turns out that they weren’t, though their tech support folks were able to reproduce it the next day. Just last week another drive vendor was in to demonstrate their latest products, a couple of which were USB-only devices. They too were surprised when I mentioned the limitation.
That’s why I decided I’d share this with you. To some of you this will be no surprise, but if you’re like most people I’ve spoken to recently, and you’re in the market for an external drive, you won’t have to learn the hard way that USB just won’t boot in OS X.