they use EV in the US a lot, where debt is in the form of bonds and is traded. Because it's traded you get a market value for the debt, like the shares.
In SGH case, it's just bank debt, with 2 lots reportedly sold (not actually confirmed), one at less than 40c in the dollar and one about 50c in the $ from memory.
So to answer your question - value of SGH debt is a guess, because there's no market for it.
SGH Price at posting:
17.5¢ Sentiment: None Disclosure: Not Held