I am about to set up another triple boot drive, but this one is connected to my MacBook with a USB adapter. I want to be sure that I do not overwrite data on my laptop's internal drive. This is the command I used for the internal drive, which was found in an Ubuntu forum, but the sizes were changed to match my drive.
diskutil resizeVolume disk0s2 45G "JHFS+" 4-Linux 20G "MS-DOS FAT32" 4-Windows 0b
If memory serves, there may have been a typo on the last character. I thought it was supposed to be a 'G', but I am very new to this.
I assume that the disk0s2 needs to be changed, but I do not know how to tell what the external drive is called. I have looked in Disk Utility and the System Profiler, but there seems to be nothing there. UNIX must be the answer.