back up external drive to time machine

meahl wrote:

yes, i tried this, but the drive I am wanting to back up is greyed out and not available to select. Is it because it is too large?

Verify you have formatted the drive to be Mac friendly ( ie r/w as GUID / HFS+ or APFS)

Erase and reformat a storage device in Disk Utility on Mac

https://support.apple.com/guide/disk-utility/erase-and-reformat-a-storage-device-dskutl14079/19.0/mac/10.15

meahl wrote:

can you back up an external drive using time machine? not back up to an external drive, I am already doing that, but rather back up an external drive.

Yes.

To exclude items from your backup, open Time Machine preferences, click Options, then click the add (+) button to add an item to be excluded. To stop excluding an item, such as an external hard drive, select the item and click the remove (–) button.

Back up your Mac with Time Machine - Apple Support