I were not able to locate it anywhere.
The film player program I used last month still had reference to 3 videos, but could not find/play them.
I think the "already mounted" was an error, and deleting the appropoiate file would help, /dev/sda1 was not the right file.
I do not have a bootable linux now, but will try next week again.
/dev/sda1 isn't really a file. One of the more difficult things about Unix/Linux to grasp is that not everything inf the files system is really an actual file, many things just 'act like one'. 'Files' in /dev are actually pieces of hardware or software emulated devices. They pretend to be files so that programs can simply open them up and read and write to the device the same way they do to a file, which makes programmer's lives much easier.
If the 'already mounted' was an error, deleting a file isn't the way to solve it. You either need to unmount the drive (or at least convince the system that the device is unmounted), or perform brain surgery on the mount table.
The command 'umount /dev/sda1' will unmount it manually, so that you can try remounting it with 'mount -t auto /dev/sda1 /mount/Partition_1'.
The file /etc/mtab contains a list of what Linux thinks are the currently mounted drives. you can see tha tlist by typing 'mount' at a command prompt, or 'cat /etc/mtab'. If that table gets out of sync with reality, you can get these symptoms. Editing that by hand is the brain surgery option. In this case, it's not that hard, as you only need to delete one line, and teh line should be the only one starting with '/dev/sda1 /media/Partition_1'
I may have pulled out the flashdrive at the wrong time last month and damaged some file.
This can cause a problem like what you're experiencing. The drive was mounted when the OS was last able to see the USB drive, so /etc/mtab still thinks things are mounted. But, the system hasn't really mounted the drive after booting this time, so it's not available from /mount/Partition_1/. Linux can get confused like this when it isn't shut down properly, which pulling out the USB drive before finishing a shutdown can cause.
This is why I think that having a bootable USB drive isn't really a good idea for continual use. It's a problem for any OS, though some recover more gracefully.