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BACKUP DATABASE { database_name | @database_name_var } 

TO < backup_device > [ ,...n ] 

[ WITH 

    [ BLOCKSIZE = { blocksize | @blocksize_variable } ] 

    [ [ , ] DESCRIPTION = { 'text' | @text_variable } ] 

    [ [ , ] DIFFERENTIAL ] 

    [ [ , ] EXPIREDATE = { date | @date_var } 

        | RETAINDAYS = { days | @days_var } ] 

    [ [ , ] PASSWORD = { password | @password_variable } ] 

    [ [ , ] FORMAT | NOFORMAT ] 

    [ [ , ] { INIT | NOINIT } ] 

    [ [ , ] MEDIADESCRIPTION = { 'text' | @text_variable } ] 

    [ [ , ] MEDIANAME = { media_name | @media_name_variable } ] 

    [ [ , ] MEDIAPASSWORD = { mediapassword | @mediapassword_variable } ] 

    [ [ , ] NAME = { backup_set_name | @backup_set_name_var } ] 

    [ [ , ] { NOSKIP | SKIP } ] 

    [ [ , ] { NOREWIND | REWIND } ] 

    [ [ , ] { NOUNLOAD | UNLOAD } ] 

    [ [ , ] RESTART ] 

    [ [ , ] STATS [ = percentage ] ]


[/code]
CommandText:='BACKUP DATABASE photographer TO DISK='+QuotedStr(SaveDialog1.FileName) + ' WITH DIFFERENTIAL ';
[code]

ولي اين مشکلت رو حل نمي کنه با يه DeleteFile ساده Backup قبلي رو پاک کن.
يه پيشنهاد. Backup ها رو با اسمي که از تاريخ و ساعت سيستم ميگيري ذخيره کن . چه بسا روزي روزگاري به Backupي در سه هفته پيش نياز داشتي.