There is a way - but it's not straightforward.

In Radia 8.1 and 9.x, OSM did not honor the FILE type SAP and insisted on retrieving the image files from a ProxyServer/Satellite.

The only way I was able to get the image from a USB drive was to:

1.  Create a standalone ProxyServer on the USB drive with the image files stored in the correct location for the static root folder.

2.  Create and assign an RPS type SAP with URI http://localhost:3466

3.  Use the OSM Pre-Download exit point to:
    - run a script that locates the USB drive (check each possible drive letter with an "if exist drive:\ProxyServer\nvdkit.exe" until you find the correct drive letter)
    - edit the ProxyServer config files to specify the correct drive letter (where required) and to set a valid "dynamic-url" (for files that may not be current on the USB drive)
    - start the ProxyServer asynchronously

4.  When OSM attempts to retrieve the image file, it will use the 'localhost' SAP to pull them from the USB drive using http.

(Note:  For performance reasons, I used the Pre-Download exit point to xcopy the USB contents to the hard drive and installed/started the ProxyServer there instead of pulling them directly from the USB)

Admittedly, I haven't tried to do this with Radia 10 and can not confirm whether this same approach is necessary or if there is an easier, more straightforward way.