I have two large media collections
c:\media
e:\media
they're too large to put onto one disk. however, I want to find duplicates. what would be the best way to do this?
You can use the Duplicate finder in Opus to search both C and E for duplicates. Unfortunately that will waste a lot of time searching outside of the Media folder on both drives, since you can't currently search multiple folders for duplicates (you can only search either one folder or multiple drives). (Don't worry, there's already a popular request to improve that, so maybe in a future version...)
If the drives are NTFS then you could instead create a temporary directory within which are junctions to the other two directories, then tell Opus to search for duplicates within the temporary directory. If you do this then be very careful when removing the junctions since many tools (including the current version of Opus) don't understand junctions and will treat them as normal directories, deleting everything they point to if you delete them. If you create junctions, remove them explicitly using the same tool you created them with. (There's already a request to improve junction support in Opus as well.)