I have an iMac with a powered ED and MacBook Air with a portable ED for the files. When I travel I create a new catalogue on my laptop and name it based on the trip. When I get home I copy the catalogue to the portable ED, plug it into the iMac and use the "Import from another catalogue" command, it merges it into the master catalogue and moves all the files of the ED. All the edits are retained. There are so many things you can do with this catalogue system.
So here's what I do and why I dedicate a drive just for LR files and photo's.
Desktop has a dedicated drive for all LR files and photo's. Before going on location, I simply clone all newer files to the location drive which is just one of about 4 other backups of JUST this data.
Take that location drive with me, shoot; everything that was on the desktop machine travels with me. Add files to it on location.
Get back, clone the other direction; travel drive to desktop. Two backups, identical clones. Very fast (only new files or changes are updated). No need for Export and Import catalog; the backup application (SuperDuper on Mac) does all this super fast. And it's brain dead easy. And I end up with multiple backups should something happen to one of the drives.
IF you keep some of the data on a boot disk and some elsewhere, this becomes far more difficult, confusing and slow. This is why if you work with multiple systems, or even just one, keeping everything on the biggest drive you can that is dedicated for just this data, makes more sense (to me) then spreading files over multiple drives or having multiple catalogs.