Immediately, while trying to make the recommended back ups, I am lost...
You write: Also, backup any wallet.dat stored in here: ~/Library/Application Support/Bitcoin/
This path I can't follow, cause there is no support folder?
The folder is called "Application Support". It should be there by default on all Macs.
When I do a search for wallet.dat files on this mac, I find those 2 files under Applications/shared.
I now only hope I have the correct files backed up?
I took the ones I found on my desktop to back up.
I can't make these files visible?
Back those up, just in case.
Another problem is that I installed a Multibit wallet which is not used.
But this program created also .dat files and I can't distinguish which files belong to multibit and which ones to Bitcoin core...
Multibit does not use wallet.dat files
What do you mean by an "external drive"?, the only drive that I can point to as external is my Time machine.
Do you mean that by "external drive"?
An external drive means anything that is used for storage that isn't any of the internal hard drives. This could be a network share, a USB flash drive, a USB hard drive, your Time machine, etc.
2:
By: "The wallet.dat file is still present but the wallet doesn't read the content of it as it looks like.",
I meant: The file wallet.dat is still physically available on this mac but the program Bitcoin Core doesn't recognize it, or cant find the file.
In this wallet.dat file the content of the wallet is stored I suppose? a piece of Blockchain?
So If this file is still there, why doesn't Bitcoin Core program read it??
Could the file be in the wrong location? I did not tell the mac to put the wallet.dat file on the desktop, Bitcoin Core done that by itself.
The time I opened the wallet and the BTC's were still there, the wallet.dat file was also located on Desktop.
So before the program found the file in first instance and then later not any more. For me hard/not to explain.
This is not how Bitcoin Core, or most software for that matter, work. Software do not just search the entire computer for certain filenames (unless they are viruses). They will only look for the filename in specific folders, folders that usually the software made itself. Because the wallet.dat files were not in the data directory (which should be ~/Library/Application Support/Bitcoin), it does not see those files.
When you first started Bitcoin Core, you should have seen something that asked where you wanted the data directory. Did you change that to something other than the default location?
Did you, at any point, go to Settings and click on the button for "Reset options"? (Don't do that now, we just need to know).
Can you please provide the contents of the debug.log file? If you go to Help > Debug Window in Bitcoin Core, you should see a button labeled "Open" under something like "debug file". Click that and it should open the debug.log file. Copy the entire contents into a post here. If it does not fit, go to
http://pastebin.com/, make a new paste, and post the link here.
With a little searching I found "application support", hidden folders..., there is nothing Bitcoin Core related in that folder.
So nothing to back up.
Multibit made me a backup file btw, called: wallet 12 backup.dat.
So there is an external hdd, it makes regular copies of the system.
Unfortunately there is no such folder in Application Support named Bitcoin.
I was not asked to where the files should be moved or in which data directory.
No, i did not reset options.
Only thing I did in between install and the failing wallet, was clean up desktop, by name.
Concerning posting the log file data here, I don't know what private data it holds, so i don't wanna put it online.
What to me looks like abracadabra could hold vital info.