It seems as if the gremlins are really out to pull the wool over my eyes. One month it’s because my C drive was full of .sst files created by Google Earth, and this time it's my external drive. This is a 500Gb Seagate drive that I use for downloads, backups of my laptop and websites, and so on. I checked the drive with Explorer a couple of days ago (Windows 7 Home edition) and found that there was only 35Gbs of free space on the drive. The last time I recall checking there was far more. I ran JdiskReport and WinDirStat on the drive and each told me that only 119Gb was being used. Where was the discrepancy?
It took me a while to work it out, some research through Google, unhiding my hidden files and so on. Then I realized what it was – there, in a folder named USER-PC. My laptop is set to backup every Sunday evening – and there were backups going back about 19 months, each measuring as much as 10Gbs. Deleting all but the latest three of the backup sets solved the problem. There is still a bit of a discrepancy between Explorer and other programs – apparently this could be due to the method Explorer uses to calculate used disk space – I have no intention of going into that.