When an idle disk block is allocated to a file, two metadata updates are required: U1: change the corresponding bit in the bitmap to indicate the block has been allocated, and U2: link the block into the file's block index (reached from inode) so that the block can be accessed via file I/O. An OS with deficiency may not support transaction-style execution of the two updates. That is, a system crash or power failure may take place after one update is completed but before the other one starts. When the system is restarted and resumes its execution, there can be a system mistake due to the unexpected incident. (1) What's the mistake scenario if U1 takes place but U2 doesn't? (2) What's the mistake scenario if U2 takes place but U1 doesn't? (3) Which mistake is less serious? Why?