[DNSB]

For what it may be worth, I just set up a Kobo Sage and copied 5,000 books to it in one pass. The import process went without issue. A couple of items is that those books have all passed epubcheck before being moved from my calibre Intake library to the main library and that I left the Sage connected to the computer/power until the import was done. Lucky? Possibly. Other people have reported issues copying over more than a few hundred books at a time. For them, copying in batches seems to work.

The only time I put my entire library on an ereader was a Kobo Clara HD with a 128GB card installed. It crashed around 6,000 books imported. I then triggered the import again from NickelMenu. Crashed around the 11,000 mark. Triggered the import again and it completed the 16,302 book import. An interesting experiment but since the database was quite a bit larger than that available RAM, searches and other operations slowed very noticeably.