if you want to be able to use both the laptop and the desktop computers with LR at will, you will need your catalogue files to be on an external drive, along with your images if you need to export high res on the go.
I would try to get a fast, but not necessarily huge, drive with a fast interface say Thunderbolt or USB 3 and use Smart Previews. This will give you at 2500 pixel long edge proxy to work from, and as long as you are only exporting images below that size you would not need to hook up the drive with the actual images. This would be all that you would need to have to run LR on the go. You could import files to that drive also while on the go, so that you can edit and export at any size. Then after the initial work when you get back to base you transfer the image files to the archive disk. Remembering to create the smart previews of course.
I would only put the catalogue on one of the internal drives of the desktop if you are not going to use LR on the laptop. Again in that situation you are going to need to have smart previews if you don't want the external drives with the RAW files to be plugged in. One caution about using smart previews for all your images is that they will take up a lot of space on the drive. I no longer have a laptop, so don't need to worry about using SP's, for running LR on multiple machines, even so my LR catalogue file with some 42600 images including all the virtual copies etc. occupies over 19 GB along with about 44GB of previews. They are just standard and 100% previews, not smart previews, which would probably at least double that.
Remember that with LR you must back up both your catalogue, and your image files separately. Since you can specify any location for your catalogue backup I would have the catalogue on a fast external so that you can use both machines. Then I would just run the catalogue backup when attached to the desktop, with the catalogue backup saved to an internal drive. You will also need to back up your actual image files so that they were on more than one physical disk.
I suggest having the catalogue on a fast disk, since it is constantly being written to, and read from. The images files are not so important they only need reading when you load them into the Develop module, or if you do not have a preview for that file already on the disk. Using SP's for all images though would mean that you did not have to worry about having the original file available unless you were exporting high res images. So they can be on relatively slow drives.
I ran LR on my around 10 year old computer (when it died last year) with LR running from an external SATA II drive over USB 2. This was the program and the catalogue/previews all on the same drive, with the images on a series of other external HDDs, again all running via USB 2. OK it was only LR4, but there seems to be little performance difference between that and the latest LR CC. It worked, but you needed to be real patient. You could not use lots of applications of the local brush on a single image, or it would constantly restart the program, which was quite quick. Oh and it was about as quick as a snail race.
Alan
