Recently I wanted to reuse an SD card that I had uploaded OSMC on for a Raspberry Pi. The card initially only showed 600mb of space. I couldn’t figure out where the rest of the storage went. I tried reformatting and didn’t get any more space. Here’s a decent guide on reformatting:
I kept digging and found an article directly related to what I was looking for, reformatting a Raspberry Pi SD Card. Here’s a good article on using the DISKPART utility on Windows 10.