USB HDD sometimes very slow, sometimes not
System: Win7 Home Prem SP1, Dell 530 desktop, plenty of disk space.
My external Hitachi 1-TB USB2.0 drive (drive L: ) contains a series of Macrium Reflect image backups, each about 70-80 GB. Daily, an automatic routine runs overnight to create a new image and store it on L:. Once a week, a batch file robocopies the latest image to a separate USB2.0 portable drive (drive U: ) which then gets stored offsite. I also delete all but the last 2 images on L: and U: every week, just before the weekly batch run, to make sure there is adequate space.
The issue: about half the time lately, the batch file fails to complete the copying process in a reasonable time.
The image on those days is reported as "corrupt".
Also, manually copying a valid 70-Gb image from L: to U: sometimes takes about 40-60 minutes, but sometimes it takes 20 hours or more.
What would be causing such behavior? I have tried stopping all extraneous programs, but that didn't affect the behavior. Even running in Safe Mode didn't make the copy process speed up.