Core Storage encrypted startup volume, upgraded to 17A264c from a pre-release build of 10.12.6. External hard disk drive, limited to USB 2.0 with a MacBookPro8,2. After the upgrade I used Recovery OS Disk Utility to unlock the encrypted volume, then Terminal to perform a dry run of conversion to APFS. No problem with the dry run so I proceeded with a proper run. Before conversion began: Error -69853: boot file not written The earliest reports of that Core Storage-related error were in 2011. A 2014 blog post unicodemonkey's tumblr — Error: -69853: Boot file not written mentions third party drivers and the kernel extension cache. There can be no such driver issue with Recovery OS so I'll take a different approach. I'll remove the volume from Core Storage world (turn off FileVault 2), then try Recovery OS Terminal diskutil conversion, to APFS, of the nonencrypted volume.