George:
Cloning hard drives can be problematic for MANY reasons.

The geometry issue is just ONE of them. Under normal circumstances if the host OS that booted from the hard drive and the attendant motherboard BIOS and all programs run under such OS were designed and promised to operate in a manner to acess the disk via LSNs then a copy that was cloned from a smaller to a larger volume has a chance of working. HOWEVER, there is no assurance of this becasue of old posterity and backward compatibility issues whereby the disk may also be accessed via CHS parameters.

Another cloning issue is the need to deal with clusters that are locked out due to bad spots on the media. Todays high density hard drives are not without areas that are bad. The format process under an OS locks these areas out from use by the supporting OS by marking the allocation units as permanently used. (For example with a 12 bit FAT the cluster map is marked with an FF7 or on a 16 bit FAT marked with an FFF7). When you clone a hard drive the bad spots of the target drive may be found in spots where the source drive has live data stored. Well designed cloning tools such as partition copy software will be able to "move" the allocation unit of data to another OK spot on the target drive. In order to do this the disk clone software must be able to know what OS is involved in the partition and then know how to perform the remapping function. (On FAT type systems this involves reallocating the linked lists in a cluster chain and/or rewriting the cluster chain head pointers in directory entries).

Yet another cloning issue is the fact that with todays type of operating systems, as they are installed, are producing a bootable OS configuration that is closely related to the system hardware configuration. If you clone the hard disk of such a system and then attempt to use the clone copy in a target system that does not have an identical hardware configuration you will most likely have problems with the ability of the cloned copy to boot successfully. Its too bad but the good old DOS days are probably gone forever!!

As for the WEB site that talked about the speed issue of how fast a 8051 design can communicate with an 8051 hard drive I had to go the same thing that you could have done!! A simple google search with the search keys of 8051 IDE HARD DISK gives the link http://www.pjrc.com/tech/8051/ide/ as one of the first hits!!!

It is my honest opinion that if you desire to continue to move forward with your project to clone hard disks using an 8051 after all the comments given here then you would do well to spend a couple of days doing a bunch of WEB searches and find out all you can about the subject. And then after that is done you will have a good appreciation of more of the issues involved. Note that some of the knowledge needed to be successful in cloning a hard drive will not be found in any one collection of WEB sites or in any one or two reference books. Years of experience with the historical background of PC's, BIOS's and BOOTING procedures will be needed along with an indepth knowlwdge of the inner workings of the file systems on the cloned media.
Michael Karas