Missing 300GB harddisk, how to recover it?
Solution 1:
Bootcamp partitions require you to use Bootcamp Assistant (same tool you used to create a Bootcamp partition) to remove them and the associated Windows assets:
https://support.apple.com/guide/bootcamp-assistant/remove-windows-from-your-mac-using-boot-camp-bcmp59c41c31/mac
As Apple says "WARNING: Do not use any other utilities to remove Windows or a partition that was created with Boot Camp."
If for some reason this can't be made to work, there is an answer from a few years ago that may be helpful:
Can't delete BOOTCAMP partition