Error: is not a primary database file.

Hello

If I try to set a database online with:

ALTER DATABASE mydb SET online

this error occurs:

Msg 5171, Level 16, State 1, Line 1
E:\Data\mydb_log.ldf is not a primary database file.
Msg 5171, Level 16, State 2, Line 1
E:\Data\mydb.mdf is not a primary database file.
File activation failure. The physical file name "E:\Data\mydb.mdf" may be incorrect.
Msg 945, Level 14, State 2, Line 1
Database 'mydb' cannot be opened due to inaccessible files or insufficient memory or disk space.  See the SQL Server errorlog for details.
Msg 5069, Level 16, State 1, Line 1
ALTER DATABASE statement failed.

The path of ldf and mdf file is correct.
And if I delete the database "DROP DATABASE mydb" and attach the files, then it works..

Thanks for your help