This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Under RISC OS 3 DOS, and indeed with any other ImageFS filing system, discs are treated as one large file and ADFS applies write-behind buffering to nearly everything it does. (See the question on ADFSBuffers for more details about this.) This means while working on a DOS disc the entire disc is treated as one large open file.
However as long as a file is held open the cache is not flushed out fully till the machine is explicitly told to do so. This means when working with non-ADFS format discs always dismount them before removing them from the drive. With ADFS format discs this is not so critical, as files aren't held open during most operations on them, but it is good to get into the habit of dismounting floppy discs.