My apologies for the time to respond. I am traveling.
I suspect that one or both of two things is happening.
- By default, CC scans for new books when you connect. On some devices, and especially on some SD cards, the scan process can take so long that calibre times out. You can turn off the scan in CC's Settings / Connecting to calibre / Wireless device.
- When there are a lot of books on a device the file system can become very slow. This is especially true if the books are on an SD card. See our FAQ [Android] I have added a lot of books and now adding books takes forever. What do I do?