Mysql - lock wait timeout while updating 70 million records
Redesign.  Relational databases let you update every row in a table, but it is poor design to need to do such.  Please elaborate on what you are really doing; we can probably discuss another way to do the task.


Compute the flag as needed instead of needing the massive update.
Avoid the flag completely.
Have a parallel table with nothing but the id and flag.
Walk through the table 1K rows at a time -- takes longer to finish, but is less invasive.  (No, do not use OFFSET.)


A disk of only 20GB is very tiny by today's standards.  How much of that is your 70M table?  It sounds like you have no room to maneuver.  So, we may have to take that into account when redesigning.

Please provide SHOW CREATE TABLE; perhaps we can shrink some of the datatypes to conserve space.