How do I cat a deleted file which is still open by its inode?

How do I cat a deleted file which is still open by its inode?

I have a deleted open file. I want to save that file. I know its inode
number.
I think that sleuthtools icat would work for me except for the following.
$ sudo icat -f hfs /dev/disk0s2 3720257 > /path/to/some_new_filename
Error opening image file (raw_open: file "/dev/disk0s2" - Resource busy)