Kobo 4.4 SD card workaround:
Saw this problem after updating 4.3 > 4.4.
Try giving your SD card a disk name, the books on your card should be visible again. (so it's not the default -blank-, 'no name' or 'untitled')
You can basically follow these steps (substitute ‘flash drive’ for ‘micro SD card’ – and be sure to eject safely!
Mac:
https://kb.sandisk.com/app/answers/d...-card-on-a-mac
Windows:
https://kb.sandisk.com/app/answers/d...ZCaWZnJTIxJTIx