Comment 2 for bug 1615829

Is this a Windows 10 machine with the Anniversary Update? If so, the problem is that MS have broken something in how the see the internal storage of the Kobo device. When you try to view the device's drive in Explorer, it will display an error. For Kobo devices with an external SD card, the card is mounted and viewable correctly.

The problem here is that calibre cannot access the main storage of the device properly. That means it cannot access the database on the device. The database is used to identify the books on both the main storage and the external card. As calibre is identifying the Kobo device, but cannot read the database, it cannot work with either the main storage or the external card.

This has to be fixed by either MS or Kobo. The current previews of Win10 do have a fix and I understand this will be released in September. Kobo may also release a fix in their next firmware.

As a workaround, you can disable the KoboTouch device driver. Calibre can then see the external SD card and be able to access it. But, it will not be seen as a Kobo device. This means the extra function such as collection management, setting the series info and uploading covers, will not be done when you send books to the card.