Many owners of all kinds of direct attached storage (DAS) have complained that their external drives (firewire, USB and thunderbolt) won't go or stay asleep. This is a legitmate issue, unfortunatly it is also a macOS issue.

Thunderbolt devices are powered on and off by Apple's Thunderbolt driver.

  • If you plug a Pegasus into the wall socket (but leave the Thunderboilt cable unplugged) you will not be able to turn a Pegasus on.
  • If you remove all promise software and drivers from the MAC and plug in the Thunderbolt cable, the Pegasus will turn on. Promise software does not control the Pegasus power state (on or off).

Of course, with no Promise STEX driver, you won't be able to access the storage. And that's all the driver does. The Promise Utility allows you to manage the storage.

If your Pegasus or any other kind of external storage won't stay asleep, this is a macOS issue and the Pegasus driver/utility plays no part. No Pegasus settings determine when the Pegasus sleeps or wakes. And this is an issue for all external storage, regardless of which manufacturer.

If you do a google search you will find that many users have been trying to find ways to have their external storage sleep when they want it to, with varying success. Things tend to change with every macOS update, a fix that worked with macOS 15.1 may be broken with macOS 15.2.

There are many possible things that can wake external storage when the MAC still appears to be sleeping.

A few possibilities are...

  • Power naps, macOS wakes occasionally to check email, etc...
  • Power saver settings, which change with macOS versions.
  • Notifications.
  • Bluetooth
  • Applications.

For reference, here's Apple's web page on this issue...

Here's the MacRumours forum on the issue...

Here's some suggested fixes...

The root cause of this issue is that the MAC itself won't stay asleep, it wakes but the display stays off, and there's no end of web pages trying to help with this. A google search gives 2,930,000 results.

Curiously, Chrome is capable of keeping a Mac from sleeping or staying alseep.

Some say that it's Chrome with an open youtube tab, but I'm not sure this is necessary, many sites have embedded youtube videos.

You can tell which apps wake up your MAC from the pmset logs in the system profiler.

This is not a simple issue. But it is a macOS issue. The Pegasus wakes and sleeps when macOS tells it to.