I don’t think Linux literally waits for you to unmount the drive before it decides to write to it. It looks like that because the buffering is completely hidden from the user.

For example say you want to transfer a few GB from your SSD to a slow USB drive. Let’s say:

• it takes about half a minute to read the data from the SSD
• it takes ten minutes to write it to the USB
• the data fits in the spare room you have in RAM at the moment

In this scenario, the kernel will take half a minute to read the data into the RAM and then report that the file transfer is complete. Whatever program is being used will also report to the user that the transfer is complete. The kernel should have already started writing to the drive as soon as the data started being read into the RAM, so it should take another nine and a half minutes to complete the transfer in the background.

So if you unmount at that point, you will have to wait nine and a half minutes. But if you leave it running and try to unmount ten minutes later it should be close to instant. That’s because the kernel kept on writing in the background and was not waiting for you to unmount the drive in order to commit the writes.

I’m not sure but I think on Windows the file manager is aware of the buffering so this doesn’t happen, at least not for so long. But I think you can still end up with corrupted files if you don’t safely remove it.