Do I need to reformat my lacie 2tb rugged usb-c portable hard drive to use it with my iMac?

safehouse69 wrote:

Using it for Time Machine backup.

iMac mid 2011, 21.5” and planning to upgrade to MacBook Air when it’s released this year 2022.

Currently running an old OS system X El captain 2015.

FYI, your iMac can be upgraded to macOS 10.13 High Sierra. You can find links to various versions of macOS within this Apple article which also includes optional instructions for creating a bootable macOS USB installer:

How to create a bootable installer for macOS - Apple Support

Just make sure to have a good backup before attempting to upgrade macOS in case something goes wrong.

As for the external drive, with macOS 10.11 your only options are MacOS Extended (Journaled) if using it just with macOS, otherwise exFAT if you need to share the drive with Windows.

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

Format to apfs guid partition to be compatible with your new Mac

And how do you intend to use the drive? As a data drive or a Time Machine backup drive, or a backup drive using some other method/software?

It would help to know what OS you are currently using as well and the exact model of the Mac which you can get by clicking the Apple menu and selecting "About This Mac".

What is the format scheme now??????????????????????