depends on how the drive is formatted. I'm guessing HFS+ journaled
In that case, i would recommend an airport extreme (or time capsule if you want a little extra storage)
if it's just HFS+ NOT journaled, I've been able to get my Ubunutu server box to read and write to that. as well as plug it directly into my laptop. Although my drive just has USB2, it should just work with whatever interface you have plugged in.
I use this for my music and movies, since my main mac is a laptop, i don't have to fill up my drive with music i don't need at all times, but i have access to it at home over the network, or i can take the drive with me and plug it in if i need to.
although sometimes it likes to mount on the ubunutu box read-only. a quick fsck.hfsplus fixes that. mostly this is after I've had it hooked to my mac.
Don't remember what i had to do to get it to work in the first place, if anything.
or if it's fat32, you could plug it into the windows machine and share from there, and still be able to read and write if plugged into the mac.
NTFS will only read on mac, not write.
to find out disk format, open disk utility, and click the info button for the drive. Mac OS Extended is HFS+