I attempted to update to High Sierra (I'm a year behind). It failed with “filesystem verify or repair failed.” The only option it gives me is restart, which I've tried multiples times, and I'm stuck in a loop.
I tried to boot into recovery mode and run
fsck on the volume. However, that fails with “CreateNewBTree returned -34 Disk full error.” My disk is not actually full, Disk Utility reports 33 GB free.
I thought maybe deleting some files might help anyway. How can I mount the filesystem to delete stuff?
Booting into single user mode actually boots me from a disk with "Install macOS High Sierra.app" on the filesystem, not my main Macintosh HD volume. It's like I'm stuck at a point where my boot disk is pointed to this upgrade install disk.
If I turn my computer on without doing anything, it asks for my disk password and shows my user's avatar image, and entering my password promptly jumps to the black macOS loading bar and it tries to install High Sierra, which quickly fails due to the disk issue.