Many of the Mac user having snow leopard or later on their system have faced the problem where the finder application crashes and shows the error the application finder can't be opened. This error often occurs when you try to access the file that are stored in the external drive. May be the file system of the drive is not supported or the drive is infected by virus. Sometimes the error also shows up when the the external drive is not connected properly you may reset the connection and then check. If the issue is with the drive then you may have to format it to fix the application finder can't be opened but do not worry about the lost data as it can be recovered if you take certain precautions. You should stop the use of the drive after you format it as the every file you copy to it will overwrite the previous one and the recovery will be difficult. You should first try to restore the files with the use of backup and if failed you are free to use Mac data recovery software after the application finder can't be opened is fixed.