Sperlock,
I'd just recommend the following:
1. A Firewire/USB 2.0 drive that you can either switch or share out via the Linux box via NFS or Samba. Why bother connecting the drive to both when Linux can do an excellent job of sharing it out?
You already have a desktop with very capable networking capabilities. Why not use it on the Linux box, format it for use with a good journaling file system (ext3 or Reiser4), and share it with the Linux box?
The reason why I suggest this is because you already have a Linux box. Most of these "network drives" are nothing but a small embedded Linux or Wind River system running on Ethernet, usually with a much slower (think ARM) CPU, sharing the drive out via a modified version of Samba.
If you share out a USB 2.0 or Firewire drive on your Linux box, you will gain much faster file sharing speed due to Samba, a much better file system if you use Reiser4 or ext3, and security you can integrate with your existing Linux user accounts via OS X.
You've got 90% of the solution already. No need to reinvent the wheel
.
Mitch