[Apollucas]

Hi everybody, I'm new to this forum, so apologies if this is the wrong place to ask these types of questions.

I've been using my e-reader on and off for a while, using Calibre to sync my libraries on the device itself and on my PC. Not every e-book I own is stored on my e-reader due to storage space.
Just today, I ran into an issue. I sent a bunch of books to my device, which worked fine since I could see the new books on my device. However, when restarting Calibre and/or when plugging my e-reader back in, I didn't see the books on the 'Device' tab on Calibre, as if I didn't send the books at all. I'm assuming I messed up something with the files on my device when updating koreader or whatever, but I'm really not sure.

Does anyone know how to fix or replicate this issue so I can avoid it in the future?
Thanks in advance,

Lucas