External drive, data not accessible in Windows

The problem:
I got the "You need to format the disk before you can use it" message in Windows Explorer, after connecting an external drive.

The story:
The wrong thing I did. A different drive was connected and the computer was in sleep mode. I disconnected (unplugged) the drive and connected a new one, then returned the computer from sleep mode to normal. The new drive's data was then inaccessible and the above message appeared. Drives are NTFS formatted, and the same model.

The panic:
I disconnected and reconnected the drive with no good results. HD Sentinel assumed the drive was in perfect shape by reading the S.M.A.R.T data. Looking for a fix I tried TestDisk and Find and Mount, but these weren't particularly useful in this case. Afterwards, I downloaded the Parted Magic ISO and burned it to a CD. I was able to copy some important files from the drive after booting with the Parted Magic disc. The drive was easily accessible that way.

The fix:
Ultimately, I said I'll give Check Disk a try, to fix the problem. And it worked. Check Disk took a while to fix the errors, but after that, the data on the drive was recognized by Windows again.
In my case, I also had to change permissions for the files and folders on that drive, to have access to them, when drive is connected to other computers running Windows.

To run Check Disk press the Windows Start button (or press Win+R on your keyboard). Then in the Search box, type "cmd" and press the Enter key. In the cmd window type "chkdsk /f E:" (or chkdsk /f /r E:) where "E" is your drive letter, then press the Enter key and wait for the disk repair process to finish. You can close the cmd Window after Check Disk finishes its work.
Afterwards, you may have to disconnect and reconnect the drive to notice any change.

Note:
I assumed that the problem was not caused by a hardware failure (good S.M.A.R.T data, no unusual noises). This may not be the case in your particular situation, so use this info wisely.
Also, I can advice you to connect the external hard drive using shorter cables when possible.