access denied to some .pdf files moved to another drive

After the first of the two 120GB drives on my laptop started to get full I
started storing some files on the second drive. Currently I have over 2GB
free on the first drive and am using 12GB of the second drive. All the video
files I have moved to the second drive work fine. However most of the pdf.
files that I have dowloaded and moved to this drive give this error message
on trying to open:
____
Adobe Reader

There was an error opening this document. Access denied.
____

I have also tried to copy the file back to my first drive, but get this
message:
____
Error Copying File or Folder

Cannot Copy "*File name here*": Access is denied.

Make sure the disk is not full or write-protected
and that the file is not currently in use.
____

As far as I can see the disks are neither full, write-protected and th files
are not in use (I have an unlocker programme and it verifies this).

Any idea what is happening and how to sort it?

Thanks in advance, it is very much appreciated.