I have a toshiba 1GB portable hard drive that cannot be read when attached to PC USB port. I have tried disk management to initialise the drive and get the I/O error. The drive reports as working in device manager and has "unallocated space on it, all 900 plus MBs. It has data on it that I wish to recover. I believe that the connection may be at fault, however cannot be sure. I have tried the drive on three computers, running Windows 7 and vista.
If there is no obvious physical defect, then you'll likely need to download a data recovery software and free version is preferrable, here is such a tool: http://www.freenew.net/windows/icare-data-recovery-soft... use it to scan the drive for lost data if it fails seek the services of a professional data recovery company.
My solution and I don't know if it works for everyone, but its worth a try before you give up. I tried connecting to many PC's and laptops with the same error "You need to format the disk in drive J: before you can use it". I also attempted data recover or disk partition software. What worked for me is luckily a few weeks before I bought a Synology Diskstation personal server. The server has its own propitiatory OS, not Windows. So when I connected the hardrive to the server it read it with no issues at all. I quickly backed up my data to the server and believe the portable hard drive might be in the stages of becoming defective so I will discard.
So try connecting your drive to anything similar (Drobo, mybe linix computer) that does not have a windows OS. A boot error might have occurred and that's whats keeping the drive from reading. But from now on I will definitely plan automatic backups! Hope this helps anyone out there with the same issue.