Rename or edit save-as files in nested combine mounted drive files causes the "Error renaming temporary file: Input/output error:"

I see what you mean. The VFS with --vfs-cache-mode writes or full will let you create the file in the root, but it can then never upload it.

This problem isn't unique to the combine backend - if you made a local backend with a directory you couldn't write to, you'd have the same problem.

I'm not sure how to solve this - in general you don't know if you can write to a directory until you try.