Can't copy folders to external disk drive because....

I have two hard drives, one for OS files the other for data files. For OS files I use Macrium to back up the drive. For data files I copy the contents of the drive onto an external hard drive. I firstly rename the folders on the external drive, e.g. from "Music" to "Music old", then I copy the Music" folder from my PC to the external hard drive. Then I delete "Music old". That's because my external drive has enough capacity for both "Music" and "Music old", and I don't want to wipe the backup disk until I've copied everything over just in case I lost the source data during the backup.

At least, that is what I always used to do until recently.

Recently I've run into a problem. When I try to copy "Music", I now get an error message saying "Music old already exists on whatever external disk, do I want to continue" then have the option of merging or not continuing. I don't want to merge the existing external drive folder, I want to make a second, newer copy of the original folder. And I'm not even copying a folder called "Music old" onto the external drive since the original that I'm backing up is called Music, and on the backup disk I want it to be called Music too. I've tried dragging and dropping, and right clicking, copying and pasting, but no luck. Am I missing an obvious step somewhere, have I misunderstood something really dumb about how to copy folders between drives, or is Windows doing something nasty to me? I should add, there are no hidden folders on the external hard disk.

Sounds very odd. Did you figure this out?

Not yet, that's why I posted the question ;-)

That's very nasty indeed.

Did you try renaming the folder on the external drive?
Like from "music old" to "nothing to do with music wink wink"

If your copy then works, you should be able to figure out how to simply rename things.
And if it works, your copied folder should be named "music old" on both drives after your copy.
That's what puzzles me. Why the copy function sees "music old" on both drives.
Never seen anything like that, and I've done a LOT of file work.
But renaming is all I can suggest.