M350 Now will not take firmware update

If you can reformat from the Clip’s Settings, that might clear away any issue.  (Note:  it also will erase your content on the Clip–move your content to your computer first, if you want to save it.) 

Also, you may find manually installing the firmware easier (I tend to avoid the Sansa Updater, personally).  Instructions are in the firmware upgrade sticky thread at the top of the forum.

Good luck!