Resizing existing partitions on an external USB2 HDD in Mac OS?

This is what the command line diskutil help gives you about the resizeVolume verb. As the last line states, it only applies to HFS+ volumes.

Disk Utility is the graphical front-end to diskutil command

diskutil resizevolume

Usage: diskutil resizeVolume MountPoint|DiskIdentifier|DeviceNode size

[part1Format part1Name part1Size part2Format part2Name part2Size

part3Format part3Name part3Size ...]

Non-destructively resize a disk. You may increase or decrease its size.

When decreasing size, you may optionally specify new partitions to create

to fill the newly-freed space. Specify these new partitions as in the

diskutil partitionDisk command. A size of zero will cause a grow fit-to-fill.

Ownership of the affected disk is required.

Valid sizes are floating-point numbers with a suffix of B(ytes), S(512-byte-

blocks), K(ilobytes), M(egabytes), G(igabytes), T(erabytes), P(etabytes),

or (%)percentage of the total size of the whole disk.

A size of "limits" will print the valid range for the current conditions of

the file system and room to grow up to an immovable object (next partition).

A size of "R" for the target partition will resize it to the maximum

possible; "R" cannot be used for the size of new partition triples, if any.

resizeVolume is only supported on a Journaled HFS+ file system.

Would you make a screen shot of the Disk Utility Partition tab so people can see what you are seeing? Make sure you cover any personal information using Preview. Post the screen shot in a Reply using the camera icon or you can drag the screen shot into a reply. Copy and paste doesn't work.

Screen shots

Try using Mountain Lion's Disk Utility. It is easier to work with.

I read that. How about a screenshot of the Mountain Lion Disk Utility?

Thanks. You are going to have to delete the All Fat 32 partition. Then erase that area. Next drag down the Mac partition using the /// at the bottom right until you get the desired size. Then add the All Fat 32 partition back. I know that isn't what you want to do, but there is no way to reduce the bottom partition that allows the free space to be added to the Mac partition.