Yes.....you will have no problem dropping mono tracks into the 6 stereo tracks on the DP-24. Refer to the manual.......but the process is VERY simple.
As for using the 128GB card you have......you will not be able to put that card directly into the DP-24 and read / import from it if it has not been formatted for the DP-24 first......as I recall. Assuming your 128GB card has wav files on it that you want to import..........you can just import those files to the DP-24 using a PC and the DP-24 USB connection to that PC. Again.....the process is very simple and the reverse............moving your DP-24 song files to a PC.......is equally simple. Back up and storage of those files can be done a number of ways.
Just A Song Writer..........
Bookmarks