I belong to those that don't have their entire library on their ereader. I use Calibre to manage it too.
My eLibrary is relatively small anyway (but if I continue to download as many freebies as I do at the moment, it'll reach those proportions eventually lol) and I only have 3-4 books on my device at the moment. I upload one, read it, then upload the next. I could upload them all since my tablet has enough space, but I don't see the need. The number might grow just because I'm too lazy to delete the read ones from there, but that's the only reason. It's (or was) the same with my Kindle - I charged it on my computer anyway (never bought that adapter thingie), so I changed the books on the device then and there.
I guess if I were to travel somewhere I would simply plan accordingly. Of course it might happen that I wanted to read THIS EXACT book that I forgot to upload - but then I would simply have do with the ones I had with me.
