Well, I tried this and it seems to work.
-In the 'Hard Disks' setup under F12 I already have an ACSI hard drive image set up with three drives, C, D ,E, on it. Hatari boots from drive 'C' of this drive image.
-In Linux, I made a new folder called GEMDOS and in this folder I placed another folder called 'F'.
-In Hatari / F12 / hard disks I went to 'GEMDOS drive' and browsed to the GEMDOS folder (Not the 'F' folder) as the target.
-On the Hatari desktop I highlighted the existing 'E' drive icon and then went up to 'Install new drive' and in the box which appeared, edited the drive letter to 'F' and the drive name to GEMDOS. This created a new disk icon, drive F - GEMDOS.
-I saved the desktop with this new icon on it, then went to F12 / save configuration and saved the new setup with the default name, hatari.cfg
On exit from / re-entry to Hatari I found that I now had the original three hard drive icons, plus the new 'F' icon. As an experiment I copied the contents of drive 'E' to drive 'F' and that appeared to work OK.
On exit from Hatari, back to the Linux desktop, I found that I could now view the files copied from the Hatari drive E to drive F in the folder GEMDOS/F.
Is this how it should work, or have I made any terrible mistakes? I was basically just guessing my way around.