As James says, you've got no chance at all - especially now you've formatted the infected computer.... Some variants of this virus stored the encryption key on the computer and you could have used some free tools to try and find this key - admittedly this would still have been unlikely (I have done it succesfully twice out of 100+ infected computers). It depends on the variant (there are many) - anyway - only applies if you still have the original infected computer in its infected state.
The other way of getting the stuff back is using Shadow Explorer - depends on the operation system and the settings - no chance anyway now you've formatted your computer.
The only other way is to pay the ransom - but that option is also not open to you now the computer is formatted.
The cryptography used involves a "key" stored on your computer combined with a "key" stored on the Russians' computer. Without both you'd need to crack a near impossible code, we're talking way beyond the Enigma Machine here - despite what you see on James Bond films - near enough impossible.
All you can do is hope that not all of the files on the external drive are encrypted. The virus won't be on there, so if you're accessing the external drive from a completely clean computer (i.e. a new, clean install of Windows that you know isn't infected) then you are fine to open the files on the external
drive (not .exe files though of course, if there are any).
Typically though it displays the ransom message only after it has done all of the files and deleted shadow copies, so that is pretty unlikely too - sorry
Some more information:
https://blogs.sophos.com/2015/12/17/the ... ryptowall/Usually this comes in via an email attachment.