mbr and partition table problems on an external hard disk (systen identifies ext as internal HDD

Are you saying thes external drive has been overwitten, formatted?

If so then

Take look at these.

FREE TOOLS

PC INSPECTOR
http://www.pcinspector.de/file_recovery/UK/welcome.htm 
File Recovery is a data recovery program that supports the FAT 12/16/32 and NTFS file systems. The current version 3.x replaces the previous version 2.x, which is now over 6 years old.

Restoration
http://hccweb1.bai.ne.jp/~hcj58401/REST2514.EXE
For 9x/NT/2K/XP. Restore deleted files that are no longer in the Recycle Bin!

Tool to check and undelete partition
Works with the following partitions:
- FAT12 FAT16 FAT32
- Linux
- Linux SWAP (version 1 and 2)
- NTFS (Windows NT)
- BeFS (BeOS)
- UFS (BSD)
- Netware
- RaiserFS
http://www.cgsecurity.org//testdisk.html
-------------------------------

NOT FREE

EasyRecovery DataRecovery is a simple, yet powerful tool that recovers data that is lost, inaccessible or deleted. It's the ultimate do-it-yourself solution for nearly all causes of data loss (except physical hardware or system problems) where your hard drive and system are fully functional.
http://www.ontrack.com/easyrecoverydatarecovery/

File Rescue 2.5
Web:http://www.file-rescue.com/

GetDataBack for FAT
GetDataBack will help you retrieve your files if the hard drive's partition table, boot record, FAT, or root directory have been damaged by a virus, formatting, fdisk or power failure. GetDataBack can even recover your data when the drive is no longer recognized by Windows.

GetDataBack for NTFS
Recover your files when the data is no longer accessible due to formatting, fdisk, virus attack, power or software failure. Get everything back even when the drive's partition table, boot record, Master File Table or root directory is lost or corrupt.
Hard Drives
Partitions
Floppy Drives
Drive Images
Zip/Jaz Drives
http://www.runtime.org/

Norton Utilities/SystemWorks
The Norton Protected Recycle Bin helps you recover deleted and overwritten files.
The UnErase wizard helps you retrieve files you've accidentally deleted.
http://www.symantec.com/nu/nu_9x/features.html

For NTFS
http://www.restorer2000.com/r2k.htm

Data Recovery
http://www.dtidata.com/
Fast File Undelete
http://www.dtidata.com/products_ff_undelete.asp

Undelete
http://www.quantumsoft.co.uk/undelete.stm

File Recover 2000
http://www.filerecover.com/

R-Tools
http://www.r-tt.com/

File Restore
If you've deleted your data and you want to get it back, you need FileRestore.
FileRestore is a simple, easy-to-use tool for recovering files that have been lost or deleted from your Windows system. Designed for Windows XP, 2000, NT, Me, and 9x
http://www.winternals.com/products/repairandrecovery/filerestore.asp

Disk Commander
In virtually any situation where you need to recover lost data from a Windows system, Disk Commander is the solution. Disk Commander performs a wide range of data recovery operations.
http://www.winternals.com/products/repairandrecovery/diskcommander.asp

DFSee
http://www.dfsee.com/
As a tool to 'UNDELETE' files that have been accidentaly deleted.
This feature is implemented for HPFS and NTFS only.

MRECOVER
http://kristenonline.com/lain/cih/mrecintro.htm

Emergency Undelete
I believe this is free
For Win2000 and NT may work on XP
http://www.c2000.com/software/#win2000

Back2Life
http://www.simtel.net/pub/pd/57588.html

Hard Drive Mechanic
http://www.highergroundsoftware.com/6.html
It's true. Even if you just re-formatted your entire hard drive, you can still get all of your data back because, unlike what most people believe, formatting does NOT erase your files! With Hard Drive Mechanic's Unformat Feature, you can restore all of your valuable data in about 10 minutes!
demo version:
http://www.highergroundsoftware.com/downloads2.htm

Filerecovery for Windows
http://www.lc-tech.com/filerecovery.asp
Filerecovery for Windows. is a safe and affordable do-it-yourself data recovery solution that is designed to recover deleted files from all types of media such as Hard Drives, Floppy Drives, SmartMedia, CompactFlash, Memory Sticks, and other types of removable media.

Undelete
http://www.executive.com/consumer/undelete/undelete.asp
You know how upsetting it is to accidentally delete a file that you needit takes just a moment to lose hours of work. Undelete file-recovery utility captures ALL deleted files including those that typically bypass the Recycle Bin. You can even recover files you purged from our Recovery Bin. With our Emergency Undelete feature, you can recover files deleted prior to Undelete being installed on your system (provided they have not been written over). Buy Undelete and you'll never again be at the mercy of lost data.

Fast File Recovery
http://savemyfiles.com/fastfile.htm
A definite must for the PC guru! It will allow the user to perform all data recovery tasks associated with disks which are not physically damaged.
 

File Scavenge
http://www.quetek.com/prod01.htm
File Scavenge is the most comprehensive, award-winning file undelete and data recovery for NTFS volumes on Windows NT, Windows 2000 and Windows XP.

Active UNERASER
http://www.uneraser.com/undelete.htm
Active UNERASER is a compact and powerful undelete utility that can recover deleted files and folders on FAT12, FAT16, FAT32 and NTFS systems. It can even restore files from deleted and re-formatted partitions.

Undelete 3.0
http://www.pcconnection.com/scripts/productdetail.asp?product_id=309884
Undelete 3.0 restores accidentally deleted files within seconds from anywhere on your system. Version 3.0 includes new SecureDelete, full Windows XP support and expanded search options.

FinalData Data Recovery
http://www1.finaldata.com/
FinalData Data Recovery Solution will recover files that have been lost by   quick formatting, FDISK, FAT damage, MBR damage or directory   destruction. It recovers data from FAT12/16/32, NTFS, and EXT2, EXT3.

------------------------------------
Or Data Recovery Services

http://www.ontrack.com/datarecovery/
Ontrack offers a full range of data recovery solutions to address your data loss needs. Unlike other data recovery companies, Ontrack provides exclusive and patented solutions that do not require you to send in your media for recovery. In situations where the hardware is functioning normally, our patented Remote Data Recovery service and EasyRecovery software solutions can solve your data loss needs safely and effectively in a matter of hours. If another company claims that there is no alternative to shipping your drive, it's because they don't offer an alternative. For situations in which the hardware is physically failing, our In-Lab services will utilize our Class 100 clean-rooms to retrieve your mission critical data.

or

FLAT RATE DATA RECOVERY PRICING INCLUDES!
http://www.i-t-s.com/datarec/datarec_pricing.htm 

or

Total Recall
http://www.recallusa.com/
Our recovery and forensic technology is used by support and call centers as well as data recovery providers world-wide.

ESS Data Recovery
http://www.savemyfiles.com/
ESS Data Recovery, Inc. has been removing barriers in the data recovery and computer forensics market ever since its inception

The famous Crazy('s)list ! The best for this job ...

LOL

Good idea, the freeware section in your famous list.

No actualy I formatted my external hard disk as a local disk. After finishing the formattting I check My Computer to see if the system is recgonising this external hard disk. Yes it was recognising it as a local disk. I even transfered a 1gb data to the disk. But when I restarted the system did not recognise my ext HDD. I checked for my ext HDD in device manager. There was this exclamation mark with yellow tag before my ext HDD. I clicked to check its properties. Device manager is saying this:
"Cannot start the device; Error Code:10."

What should I do in order to start this ext HDD either as Local or external. Please post some useful comments.

No actualy I formatted my external hard disk as a local disk. After finishing the formattting I check My Computer to see if the system is recgonising this external hard disk. Yes it was recognising it as a local disk. I even transfered a 1gb data to the disk. But when I restarted the system did not recognise my ext HDD. I checked for my ext HDD in device manager. There was this exclamation mark with yellow tag before my ext HDD. I clicked to check its properties. Device manager is saying this:
"Cannot start the device; Error Code:10."

What should I do in order to start this ext HDD either as Local or external. Please post some useful comments.

Delete it from the Device Manager and reboot. The driver may have gotten corrupted. Hopefully the drive hasn't is still ok but it might have become defective.

Actualy I did that already. I uninstalled the device from device manager. Then I clicked on scan for changes in hardeware bla bla bla.... Then it said Found new hard ware. But it takes too much time to install the hard disk. And installation is aborted saying" Your new device is not installed properly. It might have some errors" Actualy I observed one thing that my ext HDD is making a different peculiar sound. One of my friends told me that its head r/w might have crashed. He suggested me to keep the HDD in refrigator for a night and then try fixing it again. I am trying that. I am not sure if hdd head has realy crashed. There is no reason why it should crash like that. Any comments Crazy one or any top 15 experts.

>>>There is no reason why it should crash like that

Well I hate to disappoint you but the drive is a mechanical device being as such it makes it prone to failure for any reason at any time. Hard drives are pretty sturdy and can be used for a long time but they can also have undetected defects in them that causes them to prematurely fail.

From the description of the sound you say it is making seems to indicate that there is a mechanical problem with the drive. Putting it in the refrigerator may help revive it but if it does this will only in probability be temporary.

So what should I do CrazyOne!!!!! award you all points without solving my Problem. I know if there is a mechanical problem with device I cant do anything and you cant do anything. I will wait for new ideas one more week. If no one posts new ideas then you will be the winner again!!!!!!!!! :).

Also tell me is there anyway that I can find if my HDD has a mechanical problem. Any tools or utilities which can diagonise my device. May be I should try some PC Benchmarks. Any useful comments.

If the drive is not accessable then the only way to test it is with equipment that is made to test hard disk. Some computer shops have the equipment to do this.

Give one of these a shot

try using the free diagnostic utility from the vendor of your disk to check for problems with the disk.

fujitsu
http://www.fcpa.fujitsu.com/download/hard-drives/#diagnostic

IBM and Hitachi
http://www.hgst.com/hdd/support/download.htm#DFT 

Maxtor
http://www.maxtor.com/en/support/products/index.htm

Samsung
http://www.samsungelectronics.com/hdd/support/utilities/utilities_index.html

Seagate
http://www.seagate.com/support/seatools/index.html

Western Digital
http://support.wdc.com/download/
www.westerndigital.com 

>>>award you all points without solving my Problem

Just to clarify something. Sometimes "you are pretty much out of luck" or "no you can't fix that" etc is a valid answer. If the hard disk is broken then you pretty much have a situation that doesn't have an easy and inexpensive remedy and in fact the drive may not even be salvagable at all. When drives get to this point they have pretty much become paper weights. If they can be fixed the cost of the repair would problably be more than what the drive cost to buy it to begin with.

I want to ask something else. I checked for my device in my device manager and there is an yellow exclamation mark before this device. What does that mean? Is it not a resource conflict? Please reply.

What does it realy mean to say that r/w head has crashed?

Though my problem is not solved CrazyOne gave me some reasonably good answers. He deserves to be a top expert. My Thumbs up!!! for CrazyOne.

:>)