Risolto:

var Result_Diff = from c in mDcx.Cartellis.Where(p=>p.CAR_SLA_SEQ == 1)
where !(from o in mDcx.Cartelli_PCs
where o.CPC_CAR_SEQCODICE == c.CAR_SEQCODICE
select o.CPC_CAR_SEQCODICE).Contains(c.CAR_SEQCODICE)
select c;

cia cia