What is the easiest way to remove the the files that were copied to that drive?
Popping in an OS backup?
Other than that, probably WinSCP if you have it installed.
Other than the folders ZFS1 and Dockerparms (highlighted, they're part of my setup on this particular server), the remaining top level directories in an OMV4 boot drive are standard. Look for what doesn't belong.
**But you might want to backup your boot drive, in case you make a mistake when deleting files.**