It would appear that you have your join table already in place. Drive Assignments seems to link Projects to a Table Occurrence of Drives and this is exactly what you need for a many to many relationship between Drives and Projects.
If you place a portal to Drive Assignments on your Projects layout, You can use it to select as many drives as you need for the project by selecting from a drop down list or pop up menu formatted _fkDriveID field in that portal. More sophisticated data entry options are also possible.
And a portal to Drives Assignments or to projects can be placed on a layout based on Drives 4 to show all projects linked to that drive. (Use a portal to the join table if you want to modify the assignments, Use a portal to Projects if you just want a "read only" list.)
I tried what you suggested. The join table Drive Assignment was blank. I was trying to set that relationship up when I came to ask for help. I tried manually filling out two records and tried your portal. That worked properly, however, is there a way to make create a join table automatically? For instance, before I created any entries in the join table, the portal would not let me select anything. Once I added one entry, it allowed me to change the drive the something else, but i could not select anything in any other records or add or remove drives. How can I make it so that if say I made a new project, I could select a drive for it without already having a record in the join table?
Sounds like you need to enable "allow creation of records via this relationship" for Drive Assignments in the Projects to Drive Assignments relationship. Go to Manage | Database | Relationships and double click the relationship line to open a dialog where you can select that option.
You may find this demo file on many to many relationships helpful: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
Thanks Phil! That worked perfectly! I realized I had the "allow creation of records via this relationship" box checked on only one side of this relationship.