So, basically there's two basic elements for the EXS24 Sampler instrument...
Imagine you're using a sampled drum kit... each of the samples is a separate audio file that was recorded... let's say there's 16 of these separate audio files...
But how does Logic know which key on your keyboard to map which file to? And what about the various exs24 settings for each sample? Well, that's where the EXS24 Sampler Instrument patch file comes in handy. It's basically just a list of instructions telling the EXS24 which sample to place where and how it will sound. It's a very small file as it only contains the instructions and not the actual samples.
So, the EXS24 sampler instrument patch needs to know where the actual audio sample files are so that when you hit C-1 (for example) a juicy Kick will be triggered!
So, as long as you know where the samples are on your external drive then you can move them to the same location as the EXS24 sampler patch and if Logic gets needy and asks where the files are, then ask it to search again or locate them manually. After you find them once the link should be restored :)
Hope this helps