How do I remount USB Flash Drive or SD card?
There's a difference between unmounting and ejecting.
I'm not familiar with the CLI of diskutil, so I'll explain it using Disk Utility.app

When you have a mounted SD card you can unmount it by clicking the Unmount button in the Toolbar:

The SD card stays in the list, but it is faded out.
Then if you want to remount it again, just select the SD card and click Mount: