undo remove disk on time machine

IMO, Apple should improve the "Time Machine" backup and allow users to recognize a disk previously forgotten.

There is, just add it back. It will recognize the previous backups.

is there a way to "mount" the external disk again ?

Not if it is damaged or otherwise corrupt. Being removed from Time Machine does not prevent it from being mounted.

Does it show up in Go > Computer (cmd-shift-c) in Finder?

Usual reason for not recognizing a target is a cabling or hardware error. Failing HDDs tend to be flaky and drop off.

Re-add the disk, and try again. Given just a few backups have gone through, erase and start over, if necessary.

And send your feedback to Apple, as they may or may not see it here: Product Feedback - Apple

Cronos_4192 wrote:

i've tried to reconnect all the cables, and restart the computer.

Still not showing the HD.

I will try to get in touch with apple.

Tks!

Reformat the HDD using Disk Utility.

There are HDD diagnostic tools. DriveDx was my go-to here, but that seems unavailable. (Some alternatives.)

Even new HDDs can fail, and older HDDs are increasingly prone to failure as they age.