I've had the Nook Simple Touch since Christmas (the last in the family to get a Nook, so call me a Luddite!), & use it in a number of contexts. First, I use it as an eReader for books from B&N: no problems. Second, use it as an eReader for other ebubs (Google, etc.): no problems. Finally, I use it as an adjunct storage/reader device for PDF files needed for multiple purposes: these may have caused my problem(s).

For quite a while the "My Files" listing of files on the SD card was showing "unrecognized files," or "cache" files, or "blobs" which consumed multiple pages of the file listing, WHICH COULD NOT BE DELETED. Finally taking some time to look into this, I found the following:
  1. The files were not in the "My Files" file structure, visible or hidden in attributes
  2. There is a directory structure put on the SD card that has as its parent "Android" with a number of sub-directories
  3. One of them is the "My Files" directory structure displayed by the Nook in the Library function
  4. Another is under the file hierarchy labelled "data"
    • The first directory in which to look is "com.cooliris.media" (having numerous sub-directories hung off "cache")
      • These "cache" directories have been populated with a number of "index" & other files
      • Delete the files in these directories
    • The second directory is "com.google.android.apps.genie.geniewidget.new s-content-cache"
      • Mine was empty, but you may have stuff in here, depending on what you've tried to download
      • If you find stuff in here, you can delete it with impunity (at least in my opinion)
    • The third is "com.google.android.apps.maps"
      • There are a number of sub-directories: cache; debug; testdata
      • If you find stuff in here, you can delete it with impunity (at least in my opinion)
  5. Disconnected the Nook, re-displayed "My Files" on the SD card, & everything was gone.
For those having these symptoms (unrecognized files, cache files, blobs, etc.), following the above works. For those having doubts, I'm reasonably proficient in most OSes, & wouldn't advise anything that might have undesirable consequences for anyone. Don't 'root' your Nook, don't panic, don't lose your enthusiasm for the Nook. There are solutions to all problems, & this is one problem we (the users) can manage quite easily on our own.

Keep reading!