On Apple Computers, removable disks, and deleting files...
While Apple's Macintosh line of computers has always been well-known for its intuitive, easy to use interface, one issue has plagued it since the beginning and doesn't look like it's going to be solved any time soon. In fact it's probably getting worse due to changing user habits.
A computer is not very useful in isolation. You need to be able to get programs and documents on to and off of the computer or it's all just so much impotent data. Today the primary means of sharing and exchanging information is the Ethernet network, in most cases the Internet. But it isn't always the best way to get something from point A to point B, since not everyone has their own publicly accessible server on the internet to use as a transfer point. And e-mail can only get you so far due to attachment size restrictions and the fact that most residential ISPs cap uploads to a much slower speed than they allow downloads. Downloading a 100MB file through a fat 300kbps pipe isn't so bad, but uploading it through a 50kbps straw can be intolerable in today's fast-paced, instant access, ADHD world.
So we still need removable media, whether it be a floppy disk (remember those?), USB jump drive, or portable external hard disk. In most cases these are very convenient ways to get large amounts of data from one place to another, but Apple seems to have not quite thought out fully how users interact with them.
What few people realize is that the Apple decided a while back that removable media should have its own Desktop and Trash Can, just like the computer itself does (note that in OS X the removable disk no longer appears to have its own Desktop†). This sounds like a pretty good idea on the surface, but the distinction between where the computer's Desktop and Trash Can ends and the removable device's begin is, by design, fuzzy. To the user, it appears that there's only one Trash Can and one Desktop, and the user will naturally assume that it's the computer's.
†The move to a UNIX filesystem has instead shifted this to users, so each user has a desktop in ~/Desktop, and files dragged from removable media will always be copied there.
— thanks OberonDarksoul!
Most people got their first clue that something deeper is going on when they put a floppy disk into the computer, and dragged a file from the disk onto the computer's Desktop. So far so good? The file moved, right? But eject the disk and the file will disappear from the Desktop! Because what you did was you dragged the file onto the floppy disk's Desktop, not the computer's! Likewise if you dragged the file into the Trash Can, if the Trash Can was empty before it will adopt the familiar "there's something in me" bulge, but upon ejection of the disk the Trash Can will be mysteriously empty. Again, you've put the file into the removable disk's Trash Can, not the computer's. Compounding the problem today is that users tend not to empty their Trash Cans as often these days, because with modern, massive hard disks saving that extra 50k of space by deleting your Word document isn't that important, so there's no telltale bulge transition to tip you off.
The way the interface works when you're dragging files around is actually pretty intuitive, in general. If you drag the file from one folder to another on the same disk then the file will be "moved". But if you drag it from one disk to another, it will be "copied". You can override this behavior by holding down various combinations of shift, ctrl, alt, or cmd (Apple) keys while dragging to force a copy, move, alias (shortcut) creation, or pull up a menu asking you what to do — but generally you don't have to, because 99% of the time the OS will quietly and unobtrusively do what you expect it to do. You can double check by keeping an eye on the little sub-icon that will appear on the file's icon, no sub-icon means move, a plus sign means copy, and a curved arrow means it will create an alias.
Back to our removable disk, prior to OS X if you drag the file into the Desktop, it will by default "move" the file onto the removable disk's Desktop. If you override this behavior and force a copy, it will "copy" the file onto the computer's Desktop (which is generally what you really wanted to do). This, at least, appears to no longer be the case in OS X, the removable disk no longer has its own desktop.
But the more confusing issue continues to be the Trash Can, which has an interface that is still shared between the computer and the removable disk. You need to empty the Trash Can with the removable media plugged into the computer in order to actually delete the file. This explains why people will throw away file after file, watching them vanish from their USB jump drives, but not actually see the drive's free space increase. If you put the jump drive into a Windows machine with the option to view hidden and system files turned on, you will see the a folder called .Trashes on the jump drive that's still holding all those files you thought you deleted. By contrast, Windows will automatically remove files deleted from a jump drive (or a network drive) without moving them to the Recycle Bin first. So you'd better be sure you really wanted to delete that file, but on the other hand you'll get your free space back immediately, which is far more critical on space-limited jump drives than modern, spacious hard disks.
So now that it's explained, it's not so bad, but it's still an extra, non-intuitive step that most users wish they didn't have to go through. But hey, at least they've eliminated the distinction between "ejecting" and "putting away" a removable disk.