How to prevent all usb mass storages from mounting at boot
It depends on how you clarify 'mount at boot'. Do you mean the user login or the actual system boot...

Anyway, here are two ways you can handle this.


You can use one of the Startup shortcuts. In your case holding ⏏ during startup will eject all removable disks. To be honest, I am not 100% if your external USB drives are seen as such.
Edit your /etc/fstab file, the file systems table. This table contains info on how to mount which disk. If the file does not exist, you can create it. Manual pages are here.


At this to your /etc/fstab.

# device-spec     mount-point     fs-type     options 
LABEL=VolumeName  none            hfs         noauto