I follow the manual's instructions:
Set the USB Mode to "MIDI."
Soft thru OFF
Keyboard OFF
When turning editor on the do not disconnect the USB cable connected to the MC-808 while MC-808 Editor is running
Moving the faders results in changes on editor screen and vice versa.
BUT
when
I want to Read Patch/Rhythm set names
or
choose READ, or Sample
I get the error message: unable to read/write data. refer to manual's point 6.
The point 6 says this:
the display may indicate "Unable to read/write data."
1. When the MC-808 and the computer are not connected properly
(vague)
2. When the MC-808's Soft Thru setting is not "OFF" or its Remote Kbd Sw setting is not "OFF"
(not the case)
3. When a large number of notes are being sounded
(not the case)
4. When playing/recording a pattern
(not the case)
5. When playing/recording a song
(not the case)
6. When playing/recording a sample
(not the case)
7. While an editing operation is being carried out
(not the case)
By exclusion I remain with this vague point 1

In setup MIDI I have tried all the three combinations
MC-808 in&out ON Through in&out OFF
MC-808 in&out OFF Through in&out ON
MC-808 in&out ON Through in&out ON
(BTW, Which is the correct one?)
What can I do?