GR2100 is not a ZD21, and a ZD21 is the subject of this post.
aluminum vs brass shavings. If it's just aluminum, the liklihood of a charge pump failure is high. When charge pump fails it fails to provide enough volume and pressure to the HST's, because of the design the RH side is affected first.
If any brass shavings, that means that the wear plate(s) in the HST(s) are worn, galled, trashed--it usually trashes the entire HST, and sometimes BOTH of them. If you see that, you're probably gonna have to replace at least the RH HST and hope that the LH one lasts a while. If (when?) the other side dies, it will then fill the entire system with junk just like the RH one did, and may take out the brand new RH one with it. Seen it a few times. Too many times to be frank.
While on the subject, the ZD21, and especially the ZD25 and ZD28 were known for this. The problem with them was multifaceted. They didn't hold enough oil. The oil cooler system wasn't efficient enough. The filtration system was inadequate. The charge pump was inadequate. The failures were "mostly" in the Southern half of the USA where the outside air temps get toasty. The service intervals were too liberal, in that the manals specified I think every 400 hours, when in all actuality it should have been about every 150-200 hours at the most. With that being said, if anyone still has one of these great dinosaurs, I highly suggest taking GOOD care of it, cut the service interval in half and use ONLY Super UDT2 with OEM filter, clean the screen every time you drain the oil. Those folks that maintained their stuff or paid the dealer to service them every year (residential use)--never had any issues out of them. Those that used junk filters and fluid, and thought it'd be ok to run a thousand hours on fluid/filters not so much.