My external harddrive got damaged while I was doing a system update. When I run disc utility for repair I can't mount it anymore because of a "b-tree." What does this mean and can I save my external harddrive?
"b-tree" is short for "balanced tree". I'm surprized Apple would present such an unimportant technical term to a user.
The way a "tree" works is simple. Lets say you want to find someone by their last name so you build an index that reads for "A-L" go to room 5 for "M-Z" go to room 10. Then inside room 5 you see a sign that reads "for A-G go to room 2, for H-L room 3" Now where the "B" or "Balance" comes in they add one more little detail -- they adjust the letter ranges so that at each decision point the two groups of people are equal. This means that they have to switch the signs a lot as people come and go in and out of the system but if you do this you are always certain to have the minimum number of levels before you find the person.
How does it know the b-tree is broken. maybe a "room number" points to a room that does not exist. Maybe the letter on the left is higher than the letter on the right. Maybe some sequence of signs lead in a circle. May in the A-K room you see a Q-S sign. When the file system is mounted it looks for errors like these and a few more. Either way it can be fixed by simply scanning all the people, reassigning them to rooms and writting all new signs.
Of course on the disk "people" are files and "rooms" are blocks of disk sectors.