AmigaDOS does not allow to unmount file systems. An already mounted partition can only be reused if the new partition has exactly the same size and geometry. All other partitions will be mounted as .0, .1 etc.
However, if you remove the card from the reader, the volume icon should disappear from Workbench. It only stays if some files or directories on that partition are still opened.
That's similar to DF0 and the floppy disk icon. DF0 stays in the system all the time, but the floppy disk icon disappears if you remove the disk. It stays however, if there are still files or directories opened on the disk.
