unset last item of array
Use explode instead of preg_split. It is faster.Then you can use array_pop and array_shift to remove an item from the end and beginning of the array. Then, use implode to put the remaining items back together again.
A better solution would be to use str_pos to find the first and last _ and use substr to copy the part inbetween. This will cause only one sting copy, instead of having to transform a string to array, modify that, and put the array together into a string. (Or don't you need to put them together? The 'I need 'os_disk' at the end confuses me).