Of course, watch out for the Bot posts..
The messages are saying your C partition is so full, it can't even find 721 MBs?
It may be using the External Storage but doesn't have room for the OS on C.. An upgrade will normally need about 20 GBs to do its work. Part of that, I suppose, can be moved to the External Storage but the C partition needs space. If it is that full, the install takes much longer than it would normally need.
Maybe verify the free space you have on your partitions..