Apple today seeded the sixth beta of an upcoming macOS High Sierra 10.13.3 update to developers, just a few days after seeding the fifth beta and more than a month after releasing macOS High Sierra 10.13.2, the second major update to the macOS High Sierra operating system.
The new macOS High Sierra 10.13.3 beta can be downloaded from the Apple Developer Center or through the Software Update mechanism in the Mac App Store with the proper profile installed.
It's not yet clear what improvements the macOS High Sierra 10.13.3 update will bring, but it's likely to include bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.2.
It does offer additional fixes for the Spectre and Meltdown vulnerabilities that were discovered and publicized in early January and fixed initially in macOS High Sierra 10.13.2.
The update also fixes a bug that allows the App Store menu in the System Preferences to be unlocked with any password.
The previous macOS High Sierra 10.13.2 update focused solely on security fixes and performance improvements, with no new features introduced, and a supplemental update introduced a fix for the Spectre vulnerability.
Update: Apple appears to have temporarily pulled the macOS High Sierra 10.13.3 update, and it is no longer available for download from the developer site.
Update 2: The macOS High Sierra 10.13.3 update is once again available to download.
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Have you tried any of the following:
SMC reset: https://support.apple.com/en-gb/HT201295
NVRAM reset: https://support.apple.com/en-gb/HT204063
try to start in Safe mode: https://support.apple.com/en-gb/HT201262
Disconnect all external devices such as hard drives, DVD drives, card readers, even USB keyboard and mouse and then try to start the machine. this test is to eliminate the possibility of a faulty device preventing the computer from booting.
Use another mac to install a fresh copy of Sierra onto an external USB hard drive. Connect this hard drive to your Mac press and hold OPTION key during boot to bring up the start up manager https://support.apple.com/en-gb/HT202796, then boot from the USB disk and use disk utility to try to repair your internal disk.
"Install the beta software only on non-production devices that are not business critical. We strongly recommend installing on a secondary system or device, or on a secondary partition on your Mac."
As others have said, this number of betas is, and has been, common for OS X/macOS for many years, even back when Steve Jobs was still alive and in charge.