This probably isn't your problem, but I'll say it anyway: if your SD cards are nearly full, it may take your PC a while to read them and recognize them. Perhaps once you connect your Opus to your PC walk away for a couple of minutes to let your computer "see" the devices?
Also, have you tried removing the SD cards and connecting them directly to your PC and then adding/ deleting music directly? That may work if the read/ write problem is with your player itself. Of course, the rub here is your Opus would need to re-read your SD card(s) every time you insert them.
Other than that, I got nothin', sorry.