Recover data on Verbatim External Harddrive

I have a 1 TB verbatim external hardrive filled with important information. This morning I tried to access it and it asks me to format it in order to use it. I tried on another computer with the same problem. I do not want to loose any of this information. How can I recover the data that is on the hardrive?

I've seen several similar threads, in the last couple of days. The external drive won't show up in Windows Explorer, and Disk Management presents the disk as unallocated. Just one example: http://www.tomshardware.co.uk/foru [...] #t1904697. I'm afraid there's only more to come as portable USB drives become ubiquitous.

The underlying cause mostly remains unknown, I suspect that in some cases switching off/disconnecting the drive without properly detaching it has something to do with it. (When I myself had run into this problem, I also had bad sectors on my external drive, which resulted in RMA in the end. It's unclear whether the bad sectors caused the partitions disappear or improper detaching caused both.)

The basic principles for approaching this situation first is to *not panic*, because we can easily cause more harm than good.

Second is to not performing any writing operation on the disk.

To recover the data, which is most probably still there, there is two approach: file recovery, or a partition recovery. Since partition recovery involves writing operations, it's best to leave it as a last step, if it's still needed. (The most professional approach would be to create an exact duplicate of the hard drive data to be able to repeat different experiments.)

File recovery involves running a recovery software. If you need personal recommendation, please pm me, because I don't want to be seen as an agent. This software reads the drive sector-by-sector, and tries to build its own filesystem tree.

Unfortunately, this process has some limitations, for example, it finds long deleted file fragments too, the data of which is long overwritten. That being said, it can produce quite good results, but integrity of the restored files need to be checked.

What software do you recommend?

? please explain.