Need some help with out of memory External Harddrive

Well...I bought a acomdata external harddrive like 3 - 5 months ago. But today it is acting up. Well...I transfered a movie or the HD-movie which is about 7GB to it, and then I got the message of out of memory and asked me to clean up some space. But I have actually used only 600GB and there are still 231GB left or more. I know it is not the problem of my PC as other external harddrives work fine when I transfer the file to them. SO How can I fix this error? I don't wanna reformat it as it will lose all the files and data.

BTW...I thought it was the cloned one, those made in CHINA but I deleted some files, about 10GB and then tried to transfer the 7GB HD-movie to it, but I still got the same error message.