Is this is a MacBook or MacBook Pro? If a MacBook Pro you can get a card to hook up eSATA drives.
How portable is portable? Do you mean a real small drive or is something bigger okay? Sometimes I carry larger drives in enclosure, such as the one bay Wiebetech Silver SATA,
http://www.wiebetech.com/products/silversata.php other times I use the smaller [physically] drives.
Anyway I have edited DV and HDV on small drives, even 5,400 RPM drives, but faster is usually better. For a couple of streams things like those offered by OWC work fine. And most of the OWCs work fine on bus power. I had one or two large ones that I think required adapters (may have been just the old eSATA one). But I know without even looking that all the 80 - 160 Gigs worked without a hitch. Think the 250s also
Then there are things like this
www.g-technology.com/products/G-RAID-mini.cfm
Which is a small portable RAID (they also have small single drive set-ups.)