Help needed: X220 with dual-boo stopped booting to High Sierra after Security update 2019-001.

Hello,

I was enjoying my first hackintosh.

I created a dual boot with MacOS and Windows 7 from the same drive.

Last night an update showed up (I'm not 100% sure now, but I think it was called 'Security 2019-001' or something along these lines. I installed it and rebooted and it booted straight to Windows. I checked the BIOS Boot Priority Order settings and it was still set to use Mac OS X the first place.

The partitions are still there (3: one for Win, one for MacOS and one for EFI(?))

What should be my next steps?

CPU: Intel® Core™ i5-2520M Graphics: Intel HD Graphics 3000 SSD: Samsung 250GB mSATA SSD 860 EVO