What is the best format for an external hard disk to use on both Mac and Linux

I want to be able to read the data from a USB 2.0 external HD if the Mac it is connected to fails. How should I format it?

The Mac OS X operating system supports several variations of the Hierarchical File System (HFS) and a Unix File System (UFS). KernelThread.com describes the different file systems in detail.

Although the Mac's Disk Utility will format a volume as a Unix File System (UFS), this format is (evidently) supported only by FreeBSD. I've tried to mount UFS volumes formatted with Disk Utility in Linux (Ubuntu/Kubuntu) -- it won't mount, nor does QParted recognize the volume.

It turns out that the Mac OS Extended (HFS+) format is supported in Linux, with caveats. ArdisTech released a HFS+ Linux driver which you can download from Sourceforge. According to the Gentoo Wiki HOWTO for HFS+, you should turn off journaling. Also, support for the case-sensitive flavor of the HFS+ (HFSX) is only available in the Linux 2.6.x kernel. Also, Mac OS X 10.3 (Jaguar) does support HFSX, but such support is not visible in any of the GUI tools. For example, you can't use the Jaguar version of Disk Utility to create a HFSX volume.

Based on this, I decided to format my external hard drive(s) that will be connected to a Mac as HFS+ with journaling. In Disk Utility, this is Mac OS X Extended (Journaled). This way, I can work with the disk in Mac OS X 10.3 (Jaguar) and 10.4 (Tiger). Also, it looks like I could read this format in Linux, 2.4.x or better, as long as I turn off journaling.