If you switched the storage in the OS to adopt the SD card as the primary storage, then that would be a major problem. If not, do you have Settings->Storage->Let MobileSheets Manage my Files enabled? That is going to be key to resolving this. If that's checked, what storage location is listed under Settings->Storage->Set the MobileSheets Storage Location? If it's linking to the SD card, just change it to the internal storage (tap the SD card icon until the path reads /storage/emulated/0/Android/data/com.zubersoft.mobilesheetspro/files) and then you can remove the SD card without any issues. If you want to backup your library to the SD card, the correct way to do that is to go to Settings->Backup and Restore->Backup Library, select a destination on the SD card and tap OK. This will generate a .msb file containing the entire library.
Mike
Mike