[eq4me]

Did you do a copy via SQL or on the Linux commandline with 'cp' or some archiving tool like 'tar' ?
Check that the files you copied still owned by the user mysql.

Provided the user and group is named mysql you can change that back with:

chown mysql:mysql <filename>
or
chown -R mysql:mysql <directory> # this recursively changes all ownerships from the top directory down.

Be extremely cautionous when doing this and triple check that you are in the right directory.

Changing the file permissions can be done with 'chmod' please read the manpage carefully and dont do a chmod 777 or 666 it is as popular as dumb. As above be extremely cautionous that you dont accidently change the wrong files. If you accidently do an chmod -R 666 on system libraries or binaries youre screwed.

chmod 750 would make an directory read/writable by the owner, readable by the group and forbidden for anyone else

In short 777 would make an directory read/writable and an binary executable and read/writable by !anyone!. chmod 666 would be preventing anyone from entering a directory otherwise it does the same as 777.

If you do copy data around I would recommend that you use cp at least with the -iax options. If you do use tar be sure to add the -p option to preserve file ownerships and attributes.