Hi I have an external HDD (500GB), I've just changed its partition and every data on it is gone!! is there any recovery software to recover my data back?

You can recover data from external hard drive with asoftech data recovery program, I used the program before, quite easy to use and works well for me. you can download it from
http://www.asoftech.com/data-recovery-software-download.html

HI there,
The best solution in this case is to use a data recovery tool to recover your data on the drive
Free Recuva: http://www.piriform.com
But It doesn't work in very extreme cases though. If that's the problem there are more serious software solutions for this, like Tenorshare Data Recovery, but they aren't free(cost $20).I think it costs a bit but safer.( http://www.any-data-recovery.com/product/index.htm )
Another possibility is looking for a repair shop that does this kind of thing. Some of them specialize in data recovery and can restore the files in almost any situation(but no guarantee and cost a lot of money,usually cost $1000 more or less).
Good luck!

Kernel for NTFS is the best software to recover deleted partition of NTFS file system. It successfully recovers deleted, inaccessible file and folders from the corrupt, damaged, formatted, deleted, lost and missing partitions.
Free evaluation version of Kernel for recover deleted partition software is also available for download. You can use evaluation version to analyze the software features and functions. The evaluation version due to demo limitations does not save the recovered files and folders. To save recovered files and folders, you need to purchase Full version of Kernel for recover deleted partition software. http://www.windowspartitionrecovery.com

What do you mean by "changed its partition", formatting it or assigning a new letter?
The formatting process always would erase all the inner contained data. Assigning a letter often would help reactivate a partition or drive.
In your case, no matter what you've done, in order to get back the inner saved data, you'd better try a external hard drive data recovery freeware t restore your wanted files back:
http://freeware-fix.blogspot.com/2013/02/wd-hard-drive-recovery.html
Or you prefer to use a special partition data recovery freeware here:
http://www.freeware-fix.blogspot.com/2013/02/deleted-partition-file-recovery.html

Good luck!

Http://www.partition-recovery.com/ can scan the drive for a previous partition and restore the MBR/partition table.

You can also recover the files off the drive to a second drive with Get Data Back for NTFS
http://www.runtime.org/gdb.htm

I've used both of these programs, they both work well. Both programs have demo versions that show you what they can recover before you have to buy them.

Also Partition Magic will work also

Hope this helps

goodluck

You can try the following software http://www.cgsecurity.org/wiki/TestDisk , however, if you have already put more files onto the disk, you may not be able to recover the file anymore.
Hope this helps.