I can no longer drag and drop

With the Clip+ in MSC mode, can you see the file on the unit, cut it but not paste it back onto the computer?  You should always be able to move a file off the unit back to your computer, as long as you can see it from the computer, so that’s just bizarre.  If you’re in MTP mode, which brings in copy-protection annoyances, that’s different. 

If using MSC mode doesn’t help, I would wonder if your cord is going bad. If you have another USB-to-miniUSB cord, like for a camera or something, try that.  

And as long as you have copies of everything elsewhere, I’d suggest formatting (through the Settings/System Settings menu), putting it in MSC and reloading. 

And you’re right–the manual does not explain at all what MSC and MTP are about, only that they are available.  SanDisk really ought to make that much clearer.  Even their knowledge base article is a bunch of jargon. 

http://kb.sandisk.com/app/answers/detail/a_id/162

Here, SanDisk, is the plain English that ought to be in both the manual and at the TOP of the knowledge base article. 

In MSC mode, the unit will be seen by the computer as a removable drive, enabling manual drag-and-drop, copying,  deletion and other typical file handling. 

In MTP mode, the unit can be controlled and synched through Windows Media Player 10 or above. MTP mode is necessary for DRM (digital rights management) files such as library audiobooks and music from subscription services, and allows Windows Media Player to make playlists and synch files. 

 There, SanDisk, was that so difficult?