I too have the problem with two different dirves. The first one I noticed the problem on was a new drive I'd had pnly a week or two. I stored files on it with a Windows 7 system and then could not access with another XP system. Fortunately I was able to
go back to the first systm, save the files to another device and then format the new problamatic one.
On another older dive I've had for about two years, I had used exclusively on an XP machine and stored about 900 GB. Never had a problem. The a couple of days ago, I tried to access the data with a new Widows 7 system and was told I had to reformat. I
tried it on another XP system and another Windows 7 system and both had the same problem (Windows 7 sys to format, XP simply will not access). I tried checking the disk on the original XP system (took over 12 hours) but nothing wrond was found. Fortunately
I can still access with original XP system and am copying files to that disk I reformated (which all systems can see now); but this is taking excruciatingly long - 15 hours for the first 700GB.
Both drives were ones that require plugging in (the older one is 1 TB, the new one is 2 TB); I've not had this problem with flash dirves (128 MB to 8 GB) or portable drives (120 GB to 1.5 TB).
I would really like to find a better solution. For now, though, I am alwys going to format any new drives I get before using them.