Unable to Restore from removable external hard drive

I have been regularly backing up my C drive to both a removable external hard drive and to the ASZ on a monthly basis. In September I upgraded to TI (14) 2011 and eventually succeeded in backing up using build 5519.

Today I ran my October backups but discovered that the previous backups I had made to the external drive were no longer recognised by TI and consequently I would be unable to Restore from these backups.

Backing up to removable external drives has always been regarded as the ultimate in security and I am amazed that the latest version of TI is unable to see the drive when it is reinstalled.