Travel Technology - Suggestion for External HD shared among two laptops

I've looked at several threads and see some recommendations for external drives though none that really fall in line with what I'd like to do. I don't want a drive for travel, nor for business use. What I'd like to buy is a big drive that I can use for versioned backups, but do so with two different PC's. Having some software that would help me do this would be great, though Windows Live OneCare does a decent job of this already.

I've seen some software that simply appends and updates, but doesn't delete files when I delete files. If a file exists on Monday, I'd expect Monday's backup to have that file. If I then delete it on Wednesday, I'd expect it to be deleted in the backup I perform on Wednesday even if it existed on the previous Wednesday's backup.

I'll use the device to backup everything from .doc, .xls, .jpg, .mpg to anything else. Files may be big or small...the full range.

I also don't want a refrigerator sized unit, but do want as much space as possible in a smallish device. USB connectivity is a must.

What else should I look for, watch for, or what manufacturer does good with stuff like this?

If you have a Firewire port on your laptop, I'd go with that. USB 2.0 speeds are theoretical, not actual.

As far as harddrives, I am a LaCie person but I have good experience with Western Digital as well. They have a 500GB drive for something like $350.

What about a NAS (network attached storage)? That way both machines always have access to the drive, no need to keep plugging in.

What about a NAS (network attached storage)? That way both machines always have access to the drive, no need to keep plugging in.

I should have mentioned. One of the laptops is work, one is personal. I'm wanting to back up my personal files on my work laptop and also use the same device to back up my wife's personal / home use laptop.

If the drive is going to stay home, a 3.5" HD in an enclosure is all you need. Bear in mind that although there are several brands of drives, it appears Seagate, Western Digital and Maxtor are what you'll find. Due to their high failure rate, I'd avoid Maxtors. The USB port in my computer seems to get about 16MB/sec throughput, versus about 4x that for SATA to SATA on my 2 internal drives.

FWIW, LaCie just packages drives in their enclosures. The 500GB drive mentioned was available for $130 or so in my neighbourhood.

Anyone used one of these babies yet?

http://www.lacie.com/us/products/product.htm?pid=10971

If you go for portable drives I would try to avoid using the cable that comes with seagate free agent, it doesn't fit very well (we have about 10 in our field teams) and if you touch the drive wrong you get major drive errors.