Espresso machine only works with ground unhooked?

I'm not sure why the answer for the short-circuit answer got a thumbs-down because he's onto something as a very possible explanation for what is wrong with the appliance. You definitely have a problem with the expresso maker and it is "leaking" electrical current through the ground wire. A GFCI operates by monitoring electrical current going out on the hot wire vs. the current coming back on the neutral wire. If a difference is detected, electrical current is flowing somewhere that it's not supposed to, such as a ground, and then the GFCI trips out as a safety precaution. Your GFCI is not lying to you. There is a problem with that machine. If is new, take it back to the store and get a refund. If it's old, throw it away and buy a new one. Sorry. Good luck!!

Steve

Listen to all these answers - its what is known in the engineering world as "Shagged"

Without the earth connected its allowing current to continually be in an area of the machine where current isnt meant to be! The earth is vital as without it it can fry you like a cajun catfish. It will travel thru you and your body and keep on going untill you (which is hard to do when you are being electrocuted as your body goes into a grip spazm pending on the track of the current) let go or someone makes you let go be it by shoving you with a wooden handled broom, or as i have once witnessed when no availabel tool was availabe, a running leaping double footed kick thru the air.

Takes less than an amp, poorly placed, to stop a heart my friend - dont f8ck with it. Listen to the electricians!

Plumbing and Heating engineer (CORGI now GASSAFE) for 12 years - a good electrician is worth his/her weight in gold. Listen to them!

Im guessing you guys are in U.S or something yes?

I would concur with the other 2 answers. As long as your GFCI receptacle works fine with other appliances, you obviously have a problem with the espresso machine. It sounds like your GFCI is doing exactly what its manufacturer designed it to do (save your life!). Whoever gave 'thumbs down" probably isn't qualified to answer these questions.

There's a ground fault somewhere inside that machine.

It is faulty and dangerous to use.

Return it to the seller, get it repaired, or 'trash' it.

TAKE IT BACK!! you have a short in it and thats why it tripes the GFI.