Im not sure about the 1/2 frozen statement - BUT some friends of mine froze the meat because they didnt have tiem to grind it and when the did grind it it made a huge mess.. They had meat particles all over the walls and floor..

i can only assume that being frozen it sorta 'shattered' and went all over the place OR it got watery from being frozen.

Definitely remove any/all the sinew/fat that you can.

Brian