On (27 Jun 94) Rod Speed wrote to Keith Richardson...



 PE> Remember when my hard disk was stuffed you told me to do an "fdisk

 PE> /mbr"?



 RS> Yep.



 PE> Well I have found a book (from David Begley), which tells me about

 PE> the master boot record, and it has the partition information in it.



 RS> Well, its more accurate to say that the first physical sector on the

 RS> drive you are going to boot from has various stuff in it like the

 RS> partition table and the MBR. When you use the FDISK /MBR command you

 RS> rewrite JUST the MBR and dont touch the partition table which is also

 RS> in that sector.



 RS> The first sector also has the bios parameter block, various detail of

 RS> the drive, which OS formatted it, how many sectors there are etc.



 PE> How come you made it sound like "fdisk /mbr" was an
innocuous command?



 RS> Because all it does is rewrite the loader which is used to load the

 RS> first bit of DOS proper off the bootable drive. Its just a small

 RS> piece of code and so its completely safe to rewrite that small piece

 RS> of code. The FDISK /MBR doesnt touch the partition table or the BIOS

 RS> parameter block.



 KR> partly true, since it is impossible to write less than 1 sector on a

 KR> disk (at least all the ones that i have ever come across, although

 KR> you no doubt know of an exception) the partition table, although it

 KR> is not altered, must be read from the disk and re-written to it.



 RS> Well, thats just pedantic nit picking.



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i gotta give it to you rod, you've got a wacky sense of humour.



 RS> Sure, the only way to rewrite the MBR in a strict sense is to read

 RS> in the whole sector, replace the bytes which make up the MBR, then

 RS> rewrite the whole sector again. But in a practical sense you have

 RS> just rewritten the MBR even tho in a strict sense you have

 RS> rewritten the whole sector.



no, in a strict sense you have just updated the mbr, but in a practical

sense you have read, and rewritten the entire boot sector. meaning that

there is a small but finite chance of having a catastrophic stuff up, i

have had to recover systems where power failure etc happened at exactly

the wrong moment. rare but embarassing. the truely paraniod would keep a

copy of the boot sector on floppy just in case. i do not like boot

manager's habit of diddling with the boot sector every time you load dos

either.



                        Keith



--- PPoint 1.86


SD®¯¬x"