External Drive Not Cooperating

I have an external HD that is nearing 3 yrs old. I use it for backing stuff up and it's not considered one of my main methods of backup.

This AM, I went to look at a file stored on this drive and I noticed that I was getting an error message that went something like this: "Drive needs to be formatted in order to be used. Continue?" The drive name and all of the contents appeared to be 'missing in explorer. Checking the Properties of the disk, it read as if no data were stored on it.

After cycling it off/on a few times, the drive letter/name and contents finally showed back up. Checking the Properties tab also indicated that the drive had data on it. The kicker though was when I went to open the file, I got a message that said the file was "no longer on that drive".

So I unplugged it and brought it with me to work where we still use XP (yay) and lo and behold, it works fine. Everything checks out and it is fully operational. I've written to the HD and been able to extract files, etc.

So, the question is... what could cause Win7 to do one thing and XP to do another when it comes to a simple operation like this?

I can't think of an OS specific root cause if used to be ok on both.
You don't say if it's portable. If it is it may be on the edge of getting enough USB derived power.
Anyway it's getting old. Try CrystalDiskInfo check it's health
Download Center - Crystal Dew World
Also do the obvious
Windows file system check/surface scan (Properties>tools...)
Manufacturers diagnostic

CrystalDiskInfo will give power on hours.

Thanks for your prompt replies.

Yes this is a portable external drive connected via USB. I've recently replaced it with a newer HD in another chassis, so this one was only being used as a redundant backup drive. After suffering major data loss last year, I have 3 external drives of various vintage and size, etc. which i utilize for backups. (I run weekly backups to my newest external HD). All drivers on Win7 are as current as they can be I suppose. I keep up to date with Windows updates, etc. Nothing recently though.

I'll run the utilities and see how that goes.

Thanks again.