PreAllocaton and check free space are to solve different things..
PreAllocation is good for avoiding disk fragmentation. and it reserve the space so no other process can use it.
Before you start a copy or move process there is a function that checks to see if there is enough space on the target for your source to move into?
Either a way to have it run automatically on a copy/move or a separate function so a button or short cut can be set up to check for available space on target?
A Button to ask if target has the space of selected files from the source ? (Not sure I get that and how PreAllocatiopn solves it ?)
Or do you mean that it should check free space before copying everyfiles.. ? ( I think it does that. Hmm Not sure. There was some changes around this a couple of versions ago. might not remember it correct)
PreAllocation is something I have on my list. However all filesystem in MC are plugins and to support this I have to redesign some parts.. And I been avoiding doing that becuse of other reasons. But it will be included when I get the time to do that.