That's not true. External disks can be used on multiple machines. No problem at all to as much PC's in whatever sequence you like. Even Linux machine will work. And Mac's if it's formatted FAT32.
The mysterious thing in your post is your mention of "software for installing and formatting the disk". External hard disks (in any Windows after Windows ME, and current versions of Linux and Mac) don't have to be installed. They just work. If they need to be formatted again (usually they are already if you buy them), if you want more than one partition on it, or if you want a partition with another filesystem they come with, any OS will let you do that. So there's no need at all for "software for installing and formatting".
And then there's the issue that external disk are unreliable and error prone. So they aren't suitable at all for permanent storage of the only copy of a file, unless you're ready to lose it. They are excellent for transferring data between machines that aren't in local network (but yours might very well be if you have a broadband router). And they are nice as an (extra) backup. But don't use them as storage!
Just got new Toshiba laptop and want to transfer 280 GB documents, movies, etc to the new external HD from an
HP Pavilion DV 2500 and be able to access from the new Toshiba. Software for installing and formatting the external HD is included with the HD. What is bothering me is that I seem to recall having read an article saying always use the same USB when using an externall HD. Obviously I can't do this with two laptops involved. Can anyone offer information regarding this? Thanks.