format Lacie external hard drive

I'm running 12.6. To work with time machine in saving everything I do on the computer.

Time Machine will erase and format it APFS. All you have to do is add it as a Time Machine backup disk and Time Machine will format it. If you cannot see the drive when you try to add it to Time Machine, then you should used Disk Utility to format it as others have described.

I would first erase and format the drive as APFS to ensure that you've removed any software that may have been installed on the drive to make it "Mac ready"

please see Erase and reformat a storage device in Disk Utility on Mac - Apple Support

and then to assign the drive to Time Machine, please see Back up your Mac with Time Machine - Apple Support

To add an Illustration from very good link provided by @jefferythefrog

Note all the Important elements in the image and especially the Click to Show ALL Devices and the External Drive ( Upper Most by Name )

Barney-15E wrote:

Time Machine will erase and format it APFS. All you have to do is add it as a Time Machine backup disk and Time Machine will format it.

I kinda thought that was the case. but since I wasn't 100% positive on that, I supplied the "how to" link to erase the drive since it's a step that only takes a few seconds to apply.

What system are you running on your iMac?

What will you use the external drive for?

What type of drive is it: SSD or rotating HDD?

Answers to all questions are necessary to give a accurate reply.

BeagleBagel wrote:

I'm running 12.6. To work with time machine in saving everything I do on the computer.

Then it's APFS as the others have described in detail.