OSX can mount a hard disk anywhere in the file system and abstracts the concept of a seperate disk (same as linux, think it even uses /etc/fstab)
As such I think the only explanation is either not knowing the actual file path to the disk mount point, or file permissions.
I'd say run 'df -h' in a terminal or check in the Disk Utility. It's probably '/dev/diskx' and this path 'should' always work in Godot's project manager.
If you can confirm you've tried the mount point as a path and have rw file permissions, then I'd say it's a bug, but suspect something else is afoot as Godot should neither know nor care what a hard disk even is.
