I have a USB drive that I use with my Mac for a Time Machine backup and I will sometimes unplug it without ejecting it. Whenever I do, Time Machine isn't using it and, as far as I know, nothing else is either. (Spotlight isn't indexing either when I unplug it.) Is it a bad idea to do this? Will this do any damage to the drive, even though nothing is using it?
To prevent storage from wearing itself out from all of the minuscule writes a system may request from a disk, those writes are usually buffered and then flushed all in one go. When you manually eject a disk, you cause the system to flush the write buffer, and in doing so, ensure that all of the data that's supposed to be on the disk actually is.
If you unplug a disk without "ejecting," i.e. flushing the write buffer, you open yourself up to the possibility of filesystem corruption or lost data.