I was trying to copy a large chunk of data from my Mac to Seagate external Hard drive(500GB), but Mac kept saying that it can't copy. However copying a small files(4-10 mb size) doesn't seems to bother Mac.

I checked on the internet and somewhere it was mentioned that to copy larger file, the partition type must be MAC OS compatible.

I moved all the files/data from partition A to partition B and changed the type to MAC via disk utility. The process abruptly terminated and now I can't see any drives or data.
In short I lost everything and now when I connect my external drive, I keep saying " can't connect to external drive" with initialize option.

Does any one have any clue for this problem? Can I somehow get my files back?
I check with some service providers who can retrieve the data and they are quoting 350-400 to do this. Thats costing a fortune...
Any help will be highly appreciated.

Many thanks in advance for reading and helping me.