Hi,
Plug in the external drive and perform a full computer virus scan to ensure that the internal drive and the external drives are free from virus.
Run a full computer scan with Microsoft Safety Scanner to make sure that the computer is virus free.
Microsoft Safety Scanner:http://www.microsoft.com/security/scanner/en-us/default.aspx
Safety Scanner Disclaimer: There could be a loss of data while performing a scan using Microsoft safety scanner to eliminate virus if any.
Further, if need be then you may perform check disk on the external drive.
Check a drive for errors: http://windows.microsoft.com/en-us/windows7/check-a-drive-for-errors
Important: While performing check disk on the hard drive, if any bad sectors are found, then check disk will try to repair that sector. Any data available on that sector might be lost.
Thanks.