Data extraction from partition showing prohibition sign on boot

Hello.

I have a MacBook Pro mid 2012.

Yesterday I updated both my internal disks, an SSD and an HDD, from El Capitan to Sierra; after rebooting, the HDD started showing up a prohibitory symbol after logging in and the loading bar is almost full.

I tried everything, from Disk Utility to booting into Single User Mode and using fsck, to running data recovery apps, to trying to rebuild the OSX volume - but got nothing.

The disk doesn't have any physical problems, itself, seeing as both the Windows and Linux partitions on it still work and boot. Also, checks with other apps rule that out, too.

My question now is: is there any way to make that OSX volume bootable or mountable again, so I can access my files for a backup? I have 100GB+ of Photos Library that I'd be pretty angry to lose. Maybe I could go about creating an image, somehow, of the partition, hope my files are on that? But how, if the volume doesn't mount?

I can't fathom how this can happen after a simple update, and I also can't fathom how I never thought to backup my photo library...