The MBR is what holds the information for the file system.
http://en.wikipedia.org/wiki/Master_boot_recordIf you had a MBR virus or something tried to overwrite the mbr then that could explain why it got lost. The MBR was already loaded so when windows restarted it would have tried to read the MBR and thats why it didnt crash during.
The setacl doesnt touch the mbr, so I am curious what would have.
Also found some more info on it
http://www.win.tue.nl/~aeb/partitions/partition_types-2.htmlPeople often recommend the undocumented DOS command FDISK /MBR to solve problems with the MBR. This command however does not rewrite the entire MBR - it just rewrites the boot code, the first 446 bytes of the MBR, but leaves the 64-byte partition information alone. Thus, it won't help when the partition table has problems. Moreover, it can be dangerous to restore the boot code to its original state: if the cause of the problems was a boot sector virus, then vital information may have been stored elsewhere by the virus, and killing the virus may mean killing access to this information. (For example, the stoned.empire.monkey virus encrypts the original MBR to sector 0/0/3.) However, people who want to uninstall LILO, and do not know that LILO has a -u option, can use FDISK /MBR for this purpose.
So from the looks of it the MBR is what is the problem. You should be able to recover that, all the data is still there.
One thing to do would be to use a tool that allows you to look at the mbr
http://thestarman.pcministry.com/asm/mbr/BootToolsRefs.htmI havent had to use a tool like that in a long time so I dont know which one would be best to use.
But that free tool I posted before should be able to recover the partition. The only time the MBR is touched is if you change any of the partition info, such as size or adding other partitions, or if something tries to replace or modify it. there are some nasty MBR viruses out there that are very very hard to get rid of because they are in the mbr.
Again you should be able to recover it, I am more curious to what touched the MBR in the first place.
Shane