Help! Merge files was working up until last week - now it has stopped.

Hello all, I'm in a bit of a panic.

Crucial to my workflow is the merge option. Specifically merging two files, one with additional changes and items into a back up file of the same name.

I usually have the folder on my desktop, then back up to two hard drives.

I then work within the folder on the desktop, and once done, I simply open my two backup (external) drives, click and drag the updated file from my desktop and drop it into the same file on both my back up drives.

When I do this I get the option to copy or merge the files. This option no longer appears!

I have looked online and found some have said you can't drag and drop, only copy. Also that you need to hold the alt or cmd key while dragging the files across. I've tried this today and it doesn't work.

I have never had to do any of this anyway, I just drag new versions into old versions and I automatically get the option to merge.

But now it has stopped.

This is driving me nuts as my workflow if slowing down as I find workarounds (deleting the backup files and replacing them with the new - but this is of course isn't the safest way to work).

I've seen lots of old conversations online but no real answers.

Help!

Thanks in advance.. Running Ventura 13.7.6 -