If you plan on keeping the old HDD in the new computer, you can just use the catalog from there.
If you want to transfer everything to the new HDD you need to
copy the *.LRCAT file
copy the matching *.LRDATA folder (this is your previews)
You will probably need to remap the top-level folders as the drive letter or drive path will have changed.
Regardless of whether you copy those or not, you will want to copy your presets (dev, import, export, adjustment) from the old drive to the new one.
Don't forget any plug-ins you might be using.
I tried that, if I understand correctly. I get an error message "unexpected error opening catalog" with a button "Choose a different catalog" which I click.The location of the lrcat file is highlighted. I continue to get the "unexpected" error. I've tried renaming the folder from "pictures" to My Pictures" and have restarted the computer several times. I've also tried clicking on the lrcat file in Windows Explorer, which in previous versions of windows opens a dialog box "open with". that option doesn't appear in Win 10.
You're probably running Windows under a user account. I think you'll find that if you configure Lightroom to run as administrator you won't have any problems. Since you renamed your catalog Lightroom won't recognize it anyway. You'll have to use the File->Open to open the catalog under the new name. But you're going to probably have to run Lightroom as administrator.
Thanks to you both. I finally was able to open the catalog using a different door. I opened Lr and did the file/open catalog routine. That directed me to the lrcat file, I selected it, and the catalog opened.
With LR closed delete the .lrcat.lock file amd the .lrcat-journal file. Those where both left over from a crash of LR on one of your system.
"IF" with LR closed those files don't exist then disregard the above.