Parallels Desktop 8 for Mac is lets you seamlessly run Windows and Mac applications side-by-side

Thunderbolt limitations of Mac running Windows: a feature, not a bug. Sort of.

By John Rizzo

Macs running Windows with Boot Camp have some limitations working with Thunderbolt devices, some of which we have previously reported. For instance, a Mac booted from Windows won't sleep when a Thunderbolt device is connected. Readers have asked us how to fix this problem, but according to Apple, it's a feature, not a bug. Well, sort of. Apple says:

"Sleep is disabled in Windows 7 or 8 while a Thunderbolt device is plugged in."

Apple disables sleep because of a limitation of Apple's Boot Camp Thunderbolt drivers for Windows. In order for Windows (booting a Mac) to recognize Thunderbolt, it has to boot up with Thunderbolt already plugged in. Windows will "forget" that the Thunderbolt device is there if it goes to sleep. So, preventing sleep is a feature (sort of) because if Apple allowed the Mac to sleep with a Thunderbolt drive plugged in, when Windows woke up, it would no longer see the device. This would be like disconnecting a hard drive without dismounting it, which can cause messy problems, including data loss.

The fact Windows only recognizes Thunderbolt right after Mac startup causes other limitations with Boot Camp:

  • Windows won't recognize a Thunderbolt device if you plug it in after Windows is booted. You have to shut down the Mac before plugging in a Thunderbolt device, then restart with Windows.
  • Ejecting a device from the Taskbar doesn't get around this. If you want to access the device after you do a software eject, you still have to shut down the Mac and boot up again from Windows.
  • Thunderbolt is incompatible with the Windows 8 Fast Startup (Fast Boot) feature. Fast Startup is a kind of hybrid between a full boot (or cold boot) and sleep. As with sleep, Windows on a Mac forgets about Thunderbolt when it is shut down with Fast Startup running. If your Thunderbolt device isn't appearing when you boot, Fast Startup may be enabled. Try disabling Fast Startup and reboot.
  • If your notebook battery runs out, and the Mac goes into hibernation mode, Windows forgets the Thunderbolt devices. To get them back, connect the Mac to Power, wake the system, and then restart the Mac with Windows. (It's not the best idea to run Thunderbolt devices on battery power, though, even when booted from OS X.)
  • ExpressCards act like Thunderbolt when you have a Thunderbolt device plugged. This one's an odd one, but if you have Thunderbolt connected, Windows won't recognize the ExpressCard until you restart.

These limitations don't seem to apply to Windows running virtual machines with virtualization software such as Parallels Desktop or VMware Fusion. Also, these limitations don't appear when the Mac is booted from OS X. Apple has enabled OS X to work just fine with Thunderbolt.

So why doesn't Apple enable it for Windows under Boot Camp? My guess is that it's more work than it's worth because the issue lies deeper than what a simple driver update could fix. Unlike OS X, Windows isn't made for Mac hardware, so Windows isn't seeing what it expects. It's also possible different Mac models might require different hardware-specific fixes.

(With virtualization, Windows is running on virtual hardware, an extra software layer that delivers what Windows expect to find in conventional PC hardware.)

Given this, it could be some time before Apple comes up with a remedy. We're also not likely to find a workaround. For now, get used to rebooting Windows, or switching to a virtual machine.