Hi Kevin,
Since your problem is intermittent, I don't think this applies, but here goes anyway.
Are you saving to the root directory? If so, and the drive is formatted as 16 bit FAT, and there are a lot of files already there, you could be hitting the maximum. (As happened with us recently.)

http://is-it-true.org/nt/utips/utips69.shtml
Unfortunaly the root is the ONLY directory(folder) that has a set limit on the number of files it can hold. The root is actually a table with pointers to the actual storage locations. There is no limit for the number of entries in a subdirectory. OK. OK. What is the limit? 224 used to be the magic number which is the number of entries available in the File Allocation Table (FAT) for DOS. Later this was increased to 512 for 16bit FAT and there is no limit in FAT32. NTFS also has no limit. Long file names take more than one entry. So if one is using long file names and storing them in the root directory, you will hit the 512 max very quickly even though there are significantly less than 512 entries shown in the directory listing. Just depends on how long the long file names you use.