Saving Screendumps to another Volume

Hey everyone.

I am currently trying to change the directory for my screendumps on my Macbook Pro 2011 Late running High Sierra v. 10.13. I am succesful in changing my directory on my SSD/main drive, but simply can't figure out how to change the default directory to my other volume - my old HDD that replaced the disk drive.

I am using this command for changing the directory on my SSD:

defaults write com.apple.screencapture location ~/Path/
killall SystemUIServer

It works totally fine - swapped a few times to make sure I got it right.

I have tried the following commands to save it to my HDD:

ls /Volumes
cd /Volumes/"HDD 750GB"/
defaults write com.apple.screencapture location ~/Dropbox/
killall SystemUIServer

ls /Volumes
cd /Volumes/"HDD 750GB"/
defaults write com.apple.screencapture location /Dropbox/
killall SystemUIServer

The actual path when i type it in Finder is: /Volumes/HDD 750GB/Dropbox/Subfolder with spaces/Screenshots/ and I have of course tried this, but since it does not even work for the main dropbox folder I figured I wasn't typing the path wrong.

I've been trying to browse around for a while now, but with no luck. I am certain this possible and that one of you guys knows the answer.

I hope you can help me,

Thanks heaps in advance.