Can I hide a file/folder on a removable USB hard drive with HMF?

Yes, the folders on an external USB drive hidden with HMF will stay invisible providing that the disk name (letter assigned to the removable disk) stays the same and you view the drive under the same OS (on the same PC) which HMF was installed at.

However note that the mechanism of Hide My Folders resembles a shield: the files and folders you hide STAY INTACT AT THEIR PLACE on the hard drive but they are not visible under this OS as Hide My Folders provides a kind of “cloak” to them. 

So, you will find all the hidden files visible when you reinstall OS on the same PC or move a removable drive to another PC. As a solution you can install HMF on each PC were removable USB drive will be used and set the same mask to hide the folder on it.