Victory!
Well, I finally got the 8.1 to install (through the store without a complete PC reinstall) and it took no longer than 25 days :). So for all of you using multiboot configurations (with different partitions or even physical disks), just follow the instructions given earlier in this thread, but do not make the mistake I made (see below). To sume up:
- Make sure your W8 partition is active.
- If you have multiple disks in your PC with other OSes, disconnect all of them from the motherboard except the W8 one.
- When your computer does not boot now, be sure to add BCD data to the W8 partition so that you actually boot from the W8 partition. BUT do not use other ways of achieving this (like I did - I used EasyBCD utility to make my W8 disk bootable). You have to insert to the W8 install CD, boot from it and select the automatic fix of startup options.
4. Put your sunglasses on cos you will see a color tornado at the end of the installation, your entire screen will be changing from green to blue, to red, to pink burning deep holes into your brain (ooh, why do we have to undergo this? I love the decent gray of my Mac.). But your W8.1 update will succeed.
Alex