VirtualAcorn Technical support:
Using an external USB floppy disc drive
You can use an external drive with VirtualAcorn products to read DOS formatted floppies. You cannot use an external drive to read Acorn format discs. This is because external floppies expect discs to have 512 bytes per sector. However Acorn formatted discs have 1024 bytes per sector. So it's not a driver or software problem that means that Acorn discs can't be read, it's incompatible hardware. This isn't as bad as it sounds, as you can still read DOS floppies using these drives and Acorn machines can write DOS floppies.
The method you need to use to set up an external drive depends on which version of VirtualAcorn you have.
On VirtualA5000, VirtualRPC-SE or VirtualRPC-Adjust
To do this load VirtualRPC then press ALT and ENTER to switch it onto a window. Now go to the Tools menu, and click on the Options entry. The VirtualRPC options window will now open, click on the HostFS tab to open the HostFS options. You can now type in the name of your floppy into the Path: entry. For example.
Note the full stop (.) at the end of the line. Now type in a name for this drive in the Mount option, for example we use the name DosFloppy. Now click on the Add button, you will see the new drive appear in the list below HardDisc4. Now click on Apply then on OK. Now quit and re-load VirtualRPC-SE. A new drive icon will appear on the icon bar. This new drive will allow you to read and write to a DOS floppy disc. This means that you could copy archived applications onto a DOS floppy on a "real" RISC OS machine, then open the floppy directly in VirtualRPC and drag the contents to the hard disc. Note that you can only read and write to DOS floppies that already have a file on them.
On VirtualRPC-SA or VirtualRPC-AdjustSA
!DrivePop, which will be installed on your iconbar has an option to add a DosFloppy disc icon to your machine. Click Select on the DrivePop icon to open the window that lets you create new mounts. If DrivePop isn't loaded it can be found in HostFS::HardDisc4.$.Filing.
If your PC doesn't have a floppy disc drive at all then This Knowledge Base article has some suggestions that might help.