Brain lacks code for dealing with hard disk partitioning, and avoids infecting hard disks by checking the most significant bit of the BIOS drive number being accessed. Brain does not infect the disk if the bit is clear, unlike other viruses at the time, which paid no attention to disk partitioning and consequently destroyed data stored on hard disks by treating them in the same way as floppy disks. Brain often went undetected, partially due to this deliberate non-destructiveness, especially when the user paid little to no attention to the low speed of floppy disk access.