Some times by mistake my pc gets shut down by a family member without the Bitcoin Core client gets closed first. Once I open the client, it forces me to re-build/download the entire blockchain while the entire blockchain is already in its place. What can I do to prevent it from re-building which costs nearly a full day?
Always close core first before you shut down the machine. Let the little window say "do not shut down until this is gone" finish before you shut down. If you do not follow the advice the programm gives you its database might corrupt and will need to rebuild. On a slow machine it can take several days, but it will not download the blockchain again. It uses the local data.
I understand that I have to wait till the little window has gone before shutting down my computer. It's a family member of mine who is shutting down the computer without closing running applications first. I explained her everything and hope it will now not happen again. I always thought it was downloading the complete blockchain, thanks for pointing out it uses local data.
I have the same problem as you a couple of times. What I do now is that I always copy my "blocks" and "chainstate" folders to my external HDD every week. So in the worse case scenario, I could copy the files back from the external HDD and re-download 7 days of blocks. It does not solve the problem but at least ease the pain.
Yeah, some one adviced me to do the same. It saves you a lot time and frustrations. And beside that, you always have a copy of the full blockchain with you.