Easiest way to factory reset a hard drive?

So, I've got this 500GB external hard drive.
I used it to back up my laptop right before the internal drive kicked the bucket.
Right now, the thing is split into two partitions—about 100GB each.
What’s the easiest, most lightweight way to wipe it and get it back to its factory state—just one single 500GB partition?
The backup was made with Acronis True Image, but I don't have it installed right now and honestly, I can't be bothered to reinstall it—too much hassle. I figured there’s probably a simpler tool out there, maybe even something built into Windows.
Any recommendations?

If you're using Windows, just plug that portable hard drive into a USB port and head over to Disk Management. From there, you can wipe out all those existing partitions—once they're gone, you'll be able to create one single, clean "factory" partition of about 500GB. It’s pretty straightforward. Here's a video tutorial that walks you through it:

Easus Partition Manager is just a simple, straightforward tool for this kind of stuff.

Got Windows sorted just like @Osmi suggested—pretty straightforward stuff.