I will try to answer your questions at face value. But
you might find it helpful to tell us what your end goal is that caused you to ask your questions.
Although I don't know for certain, I think this is what you'd do to find out how much space is available on the SD card:
Run the Use SD command (Maintenance menu). I think the B2 will reboot. Then the music source will be the SD card.
Run the Scan Disk command (Settings menu).
Check the status. I forget how to do this with the remote - I think you press the Info button until the desired status line comes up on the display. Or you can check the status in the WebUI.
After you get the info you want, be sure to run the Use HDD command
(Maintenance menu) to get the music source back to the HDD normal.
If you have any questions about the status info that is displayed, post it here and we'll see what it means.
The music you Import from a flash drive normally goes to the HDD. But as indicated above, that can be switched to the SD card if you really want to, but it's not the normal way it works. The SD card is where the analog recordings go - temporarily - before they're transferred to the HDD.
If you have a need for more space on the SD card, the card can be replaced with a larger one, and the Brennan operating software can be loaded onto it before you use it to replace the existing SD card. We can advise further if that's what you want.