Epson 3865X Plus User Manual page 86

For example, if you enter a command to copy data from A to B,
MS-DOS copies the data from the first diskette you place in the
drive (which would be drive A) to the computer's memory.
Then MS-DOS prompts you to insert another diskette (for
drive B) and copies the data from memory to the new diskette.
When copying is complete, you see a prompt to insert the
original diskette (A).
Because you may often swap diskettes this way, it is important
to remember which diskette is which. It is also a good idea to
write-protect your original diskette. See "Write-protecting
Diskettes," above.
If you have a hard disk and one diskette drive, you can load the
operating system and application programs from the hard disk,
create and store your data there, and use the diskette drive just
for copying data to or from diskettes.
However, if you have only one diskette drive and no hard disk,
you need to use that drive to load the operating system as well
as any application program you are using. First, insert the
operating system diskette (the Startup diskette, for example) in
drive A and load the operating system; this copies it to the
computer's memory (RAM) so you do not need to leave the
system diskette in the drive. Then remove the system diskette
and insert your application program diskette to load that data
into memory, too. See your application program manual for
detailed instructions.
Using Your Computer 3-19