Holding down one key to copy instead of moving, is exactly what you demand/suggest!
no, not at all. copying the file on the dektop will not copy it into the boot drive (or a driver of your choice), it remains where it was.
No, it does not remain! Drag and Drop any file while holding ALT and the file gets copied to the desktop (means copied physically to your boot drive). If you drag and drop without ALT it gets moved and remains at the original mass storage.
there are many desktops, just like every volume has a finder window - but no chance to copy files into them by dragging, because you cant choose another desktop but the one of the boot drive.
There are not many desktops! There is just one desktop, look at your screen, there is just one desktop!
There are different disks, and you can put stuff from your different disks to your one desktop! So if you like to copy a file from any stick to any disk, drag and drop it to the disk (it gets copied). If you like to put any file to the deskotp drag and drop it from the disk you like to the desktop. The icon gets moved, and an entry into your disks desktop.db will be crated, but it still ramains the one and only desktop that you can see (think about my posting above, that you can create a "setup" at dektops at other machines, with every mass storage. It will be this one single other desktop if you put your stick in another computer.
If you like to copy a file from one mass storage to another one and from there to the desktop, drag and drop it to the disk, and klick again and move it to the desktop. That is 2 klicks and one move (with spring loading folders) to perform 2 completely different tasks! Perfect UI design!
making sense or "having an inner logic" is not necessarily the most important property of interface design.
Oh, it is! But I am discussing this for 20 years now with people who either think that eye candy is most important (an who are thus mixing tools with presentation) or strange physicists and mathematics brains who think, thinking in 3 more abstraction layers and typing commands is a good idea
however, you ignored the original issue: how to copy a file from an usb stick to the desktop of disk 17?
As said, drag and drop it to the disk, release the mouse button, klick once again, and move it out to the desktop.
Thus you immediatly did what you liked to do, you "saw and used" the disk where your file is now physically, and it takes about 0,5 seconds longer than to copy it to the boot drive (if spring loaded folders is adjusted to quick reaction).