I'm using an external USB box for one of my harddisks in Windows 7. At random moments, windows shows me the popup asking me what I want to do with the connected USB drive. It seems Windows somehow loses the connection to the drive and then 'discovers' it again. Sometimes it happens that the drive just 'disappears' and doesn't come back.
At first I thought it may have something to do with the USB device switching itself off after some timeout, but it also happens when I'm in the middle of a movie, playing from that harddrive, causing the player to stop. So I don't think it has anything to do with the disk being idle.
So I wonder if this is a software/driver problem, or a problem with the hardware.