Apple has, without warning, removed support for third-party SSDs in OS X 10.10 Yosemite, leaving anyone who uses one as a boot drive and has TRIM enabled with a Mac that won't boot.
See also: OS X Yosemite and third-party SSDs: Here's what you need to know
While this is unlikely to be a problem for the average Mac user, for high-end Mac users such as filmmakers and videographers running customized systems it's a serious problem.
According to Cindori Software, makers of the popular utility Trim Enabler, the issue is down to a newly introduced security feature called kext signing. A kext is an OS X kernel extension, or a driver.
Enabling TRIM is one of the best ways to maximize the life of a solid-state drive, but OS X doesn't support it out-of-the-box except for Apple's own drives, but until now it has been possible to enable it by altering drivers, although that has never been officially sanctioned by Apple.
"Kext signing basically works by checking if all the drivers in the system are unaltered by a third party, or approved by Apple," wrote Cindori Software on its blog. "If they have been modified, Yosemite will no longer load the driver. This is a means of enforcing security, but also a way for Apple to control what hardware that third-party developers can release OS X support for."
The Trim Enabler software has been updated so that it can disable kext signing, but this is a global setting, and disables an otherwise useful security feature. However, if you rely on third-party SSDs, it's your only hope.
"It is important to note that disabling the kext-signing to enable Trim is best described as taking a sledgehammer to crack a nut, and for most users it will not be worth it," writes Cindori Software. "But I have been unsuccessful in creating a less invasive method to enable Trim, as all of Apple’s AHCI SATA drivers are closed source and undocumented, which makes it impossible for me to create my own Trim driver and get it signed. This is the only alternative for enabling Trim for the moment."
The blog post also outlines how users locked out of their Macs following an upgrade can get back into their systems.
"I have to say I’m extremely disappointed by Apple with this," writes Andrew Reid on the DSLR filmmaker site EOSHD. "To have an official installation of OS-X simply stop working when upgraded from Mavericks to Yosemite is unacceptable for any user. For professional environments and pro video it is even more troublesome and it puts vital data and creative projects at risk. Is this what Apple wants to be known for? At the very least the installation should detect if an appropriate SSD is present before attempting to install and notify the user if there’s a problem. I just had a grey screen and a dead system."
Some SSD drives, such as those supplied by OWC, do not require TRIM to be enabled because they have the feature built into the drive, and are as such unaffected.
The big takeaway here is that if you are running a third-party SSD as a boot drive, and you do have TRIM enabled, then you need to take steps before upgrading to Yosemite to make sure that your drive will continue to be accessible after the upgrade.