If you're using Boot Camp on a Mac to run Windows 10, you might have trouble getting the Windows 10 May 2019 Update, according to a new Microsoft support document. Specifically, the issue affects Macs that came out prior to 2012 and newer Macs that have older versions of Boot Camp or Windows Support Software drivers.
It's an issue with the Mac HAL Driver (MacHALDriver.sys), if it's dated September 24, 2011 01:57:09 or older. Upon attempting to upgrade to the May 2019 Update, you'll get a message that says, "Mac HAL Driver - machaldriver.sys: Your PC has a driver or service that isn't ready for this version of Windows 10."
There are some workarounds though. Obviously, the best solution is to update your Windows Support Software, which you might find by checking for updates in the Mac App Store. Apple also has a support page with instructions.
That doesn't solve the problem for Macs that came out before 2012 though. Remember, those PCs can't even be upgraded to the latest version of macOS. Microsoft says that it's working on a fix though, so those devices won't be left out in the cold. The fix for the issue is estimated to be made available later on this month.