I have an Seagate 1TB backup plus portable drive (SRD0SP0). It works perfectly on my Mac. However, I can’t open it when I use Bootcamp or any Windows laptop. HELP!
If the drive is not formatted in HFS+ as suggested by the others you might find the USB cable is on the verge of failing. Also how old is the computer vs Mac? USB connectors wear out over time so it might also be the port used. So if the HFS tips don't help check the cables.
I agree with the others. The most likely scenario is that the HD is formatted in HFS+. You could use special drivers, as suggested by others, or you could reformat the drive on the Mac (using Disk Utility) in FAT32 format. This will take a little time, as you'll need to copy the files from the HD to another location prior to formatting, and then copy them back to drive, but it will allow you to use the HD on both systems without additional software.
The only major limitation of FAT32 that you should know about is that individual files can't be bigger than 4GB. This can be a nuisance with video files or DVD backups.
Since you have been using the drive on a Mac it is most likely formatted as HFS+ which is not supported by Windows. As ha14 said you will need some driver or software which is capable of accessing that file system.
one nice little tool is HFS Explorer
The one thing that does struck me a bit is that your Bootcamp Windows cannot access it. Did you use the Mac OS X Installation DVD to install the drivers within Windows? Have you checked for updates? By default bootcamp will install a HFS driver into Windows.
If none of the options above work I would assume your hard drive is formatted using the old Apple Partition Map scheme which is solely supported by Mac OS and OS X. In that case you would have to backup all the files, repartition the drive using the Master Boot Record or GUID Partition table.
Sorry! My bootcamp able to access it. I had a lot of things going on in my mind when i typed the description so i got confused.