>I want know , the minimum length of number of HD , and maximum length .
>Number of HD is always a number ? Or can be a caracter ?

HD's serial number is an unsigned long that can be stored as an integer: if negative, it should be complemented first. If you want to store its hexadecimal representation (such as reported by a DIR command), you should take that in account:

m.lnSerialNumber = -390867900 && my C drive, actually
* updated - expression's first version returned incorrect results
? RIGHT(TRANSFORM(IIF(m.lnSerialNumber<0,2^32+m.lnSerialNumber,m.lnSerialNumber),"@0"),8)

In this case, you have a 8 bytes wide character string.