Understanding the nuances of this word pair, indeterminate and indeterminable, hinges on understanding the words' parts. The root word, determine, means to establish something. The prefix in- in this case means not. Both –ate and –able create adjectives, -ate meaning having the characteristics of and –able meaning to be able.

Indeterminate, then, means not (in-) having the characteristics of (-ate) being fixed (determine): not fixed. Indefinite. Not determined. As in:

Prisoners on indeterminate sentences 'left in limbo' over parole dates

A man of indeterminate age peeks out of a cut-glass window.

For no discernibly good reason, I am back on Twitter for an indeterminate period of time.

Indeterminable means not (in-) able to (-able) establish (determine): incapable of being fixed or determined. Such as:

This equated to 1082 votes for and 102 against, with two ballots indeterminable.

However, there are those movies that fall into that rare category of indeterminable.

I said before how if B existed inside of the mold by itself, B's size would be indeterminable because it rests inside of infinity, which it can't be compared to.

Remember: if something isn't fixed, it's indeterminate. If it can't be fixed, it's indeterminable.