I am having a SATA HDD 300 GB which I am using as external HD by putting it in Case powered by USB. I was using it since 1.5 yrs and i have made 3 partitions in that. Suddenly last week my hard disk was not shown in my computer. I checked in disk management, there it is showing 300 Gb unallocated.
I tried to recover the partition but I am getting an error like " Cant read MBR", "Cant edit MBR", "Failed to initiate HDD"' etc by so many softwares.
Si I was not able to recover and also not able to make it usable till now.
It sounds like more than the MBR is hosed. You are not booting from this external drive, are you?
Standard advice is
1) Don't do anything that might write to the drive. Don't try to re-partition it.
2) Hope that the problem is with the enclosure. Take the drive out and try it directly connected to the motherboard as an SATA drive.
3) If this fails, and you need the data on the drive, try one of the many free-to-scan, pay-to-recover recovery programs that you can find on the forums. Clonezilla is free. EASEUS gives away almost-complete versions of their disk clone software.