Owners of older Macs or those with out-of-date versions of Boot Camp are being blocked from updating Windows 10 to version 1903.

Users of affected Mac devices that use Boot Camp attempting the update will see an error message warning about a problem with the Mac HAL driver 'machaldriver.sys' on their Windows 10 installation. "Your PC has a driver or service that isn't ready for this version of Windows 10," the message reads.

"No action is needed. Windows Update will offer this version of Windows 10 automatically once the issue has been resolved," it concludes. Microsoft advises it is working on a resolution and estimate it will be available in late July.

According to a Microsoft support page found by Windows Latest, the problem occurs with Macs released before 2012, as well as newer Mac models that use older Boot Camp versions or have Windows Support Software drivers with a compatibility hold in place. More specifically, the problem lies with the identified driver dated for September 24, 2011 or older in the Windows\system32\drivers directory.

As a workaround, Microsoft advises updating the Boot Camp Windows Support Software drivers could replace the driver in question and enable an update to go through. Microsoft suggests users can check for Windows Support Software updates in the Mac App Store and also links to an Apple support page dealing with installing the Windows Support Software itself.