My 8Gb flash USB drive seems to be working all right most of the times, but some folders stored on it become empty all of a sudden. I reformatted it several times successfully, but the problem still persists. I can't find any regularity in what folders and when become empty. Thanks.
Your flash driver is most likely a fake. It is a lower capacity drive, usually 1 or 2 Gb, made to appear 8Gb to the operation system. There's an easy test to find it out.
* format your drive again to make sure it's empty.
* on your computer, create a folder with one or more files just under 1Gb in tottal. Name it test1 and copy over to your flash drive.
* open test1 folder on your flash drive. All the files should be there and still readable.
* rename your orginal folder to test2 and copy it over again. Check if it's still readable.
* rename the original folder to test3 (test4, test5...) and repeat the test.
if your USB drive was 8Gb as advertised, you would have folders up to test8, all well and readable, on your flash drive. On a fake flash drive you will see that at some stage (on test2 for 1Gb, test3 for 2gb) the copy speed will dramatically increase, and created folders will remain empty. If you do the same with files, not folders, you'll have broken files.
There are special software utilities to restore USB flash drives to they real capacity, to make it real working 2Gb flash instead of fake broken 8Gb, but I don't know any of them supporting English. Apparently these tools are written by the same people who created utilities to fake USB drives in the first place?