Ubuntu Networking :: NAS Saving Existing Files Fails?

I have a really nuisance problem. I have two NAS servers - different makes - both of which work perfectly under Windows XP. I can write new files, save existing files without error. Both servers have drives formatted to FAT32 - the only option - although the OS's are almost certainly an embedded Linux.Both units are recognized by Ubuntu 10.04 - and I can write new files to them both without problem (although the directory does not refresh - I have to reload it). I can also delete without problems. If I try and save over an existing file I get an error "Unexpected error: Invalid argument" - I presume the implied delete is failing but am not sure. New files also seem to be saved with the Read Only flag set.OK - I can work round the problem by deleting the files (that works OK) and saving them anew. Works fine - but a backup is impossible as any changed file will give rise to the error.I have tucked both drives into the hosts files in /etc - although I attach by IP address (which is fixed) anyway.