GarageBand question

Reet. Get into the Garageband Prefs (Command + ,) and check the advanced tab. There should be an Audio Resolution tab. Trust me, you don't need it on Best. If you change the dropdown menu to 'Good' that might well allow your system to record longer. The difference between 24bit and 16bit recording is so minimal that for a podcast you're never gonna notice it.

I doubt his problem is hard disk space !?

The difference between 24bit and 16bit is enormous - 24bit is 256 times the resolution of 16bit.

But you are right 16 bit is more than enough for a podcast (and all music released in the last 2 decades)