I'm using UDF for my LinuxOSX setting and it's awesome.

Ok everybody's occupied with surreal suggestions, but anyway:
*UDF* is quite awesome as a on disk format for LinuxOSX data exchange, because it has a file size limit around 128TB, supports all the posix permissions, hard and soft links and whatnots. There is a nice whitepaper summing it all up:
http://www.13thmonkey.org/documentation/UDF/UDF_whitepaper.pdf

If you want to use UDF on a hard disk, prepare it under linux:
1) Install uddftools
2) wipe the first few blocks of the hard disk, i.e. dd if=/dev/zero of=/dev/sdb bs=1k count=100
3) create the file system : mkudffs --media-type=hd --utf8 /dev/sdb (that's right, UDF takes the whole disk, now partitions)

If you plug this into OSX, the drive will show up as "LinuxUDF". I am using this setup for years to move data between linux and OSX machines.

I'm using a USB Disk formatted under linux with UDF (yep, it's not limited to DVDs, there is a profile for hard disks). It can be used without problems under OSX (even Snow Leopard)
 

Actually, this thing is a biometric super-smartcard with optional root access from anywhere. This means, that you can load trusted applications into the device and have trusted access to the device from everywhere. If a couple of posters wondered about the uses of such a device, I have some immediate ideas (other from a 100mA cluster node):

• Secure access to corporate network. The blackdog would install a vpn tunnel between the blackdog device itself and the corporate network, i.e. tunneling the host completely, running the corporate apps via the blackdog. This means you can access data from an untrusted and definitely virus infested host (Internet cafe, Windows XP). keyloggers shouldn't be able to log anything interesting, because authentification is done on the blackdog.
• actually safe online banking/credit card transactions. This currently requires an external card reader and software installation. This device would allow me to make secure payments from almost everywhere, using the same kind of tunnel scheme as above. Remember all the sci-fi movies where you can pay with a fingerprint? Why should you trust their fingerprint reader, when you can have your own?
• for the overcautious: you should be able to boot from the thing, right? Into the apps!
• It would make an excellent and cheap solution for network licencing. The blackdog runs a FlexLM kind of licence monitor, licences can be upgraded through trusted channels, usage conditions could be freely programmed. Administrators are not harmed, they just plug the thing into a machine with a network connection and are done.
• An external virus scanner/a plug in remote debugger.
• Plug the device in, it starts a program that loads the current routing table into the blackdog, disables access to every network device on the host, (except for the blackdog which installes a kernel driver) and sets the default route to a blackdog controlled usb network device. What have you got? A hardware firewall.
• You can encrypt your harddisk with a really long key