FLAC files can't be opened for writing

I have approx 1200 FLAC albums on a portable hard drive that I've been working through adding artwork & Tags, prior to copying the files onto a Cocktail Audio unit; there is 260 gb of spare space still on the portable unit. I've completed 900, but now (intermittantly) get messages telling me that an Album can't be opened for writing - this happens on both my main PC and on various laptops (using Win 7 64 & 32 bit OS's). It also is now acheingly slow (30 mins or longer) to save tags and artwork when it actually does let me do this - any ideas on how I can get things back to normall would be much appreciated.

could you have a look at this thread and give feedback?
"File Cannot Be Opened For Writing"

I had a look at the link but my problem was not completely addressed.

I am using Win XP and I am not a computer expert at all. FLAC (track) titles are not showing in mp3tag 2.51 or 2.49 It may be a memory thing, or it may be a Foobar thing.

I find if I use mp3tag 2.48 I have compatability.

Could you check Tools>Options>Tags whether your the suffix of files is included in the list of file types?

In reply to:

 Could you check Tools>Options>Tags whether your the suffix of files is included
  in the list of file types?

Thank You.
That works - I can now use mp3tag 2.51

What works now?
What do you have checked?
Did you change anything in the list of allowed file extensions?
Now, do you have two entries in the list of allowed file extensions?
For example ...
.flac;.FLAC;
This would be rather unusal and lead to other questions.
Why got the standard entry "*.flac" lost?
What type is your computer system?
Does the filesystem support case sensitive filenames?
Is it possible to use FLAC files, which previously cannot be loaded, to load them into Mp3tag, without modification of the list of allowed file extensions, after changing the file extension from possible uppercased ".FLAC" to lowercased ".flac"?

DD.20120608.1122.CEST