An posix error, has nothing to do with the size of the hard drive that you are backing up to. Open the console application in the utility folder and place cut and paste the error here.
But please, please please reconsider your decision on a 160 Gb drive. You want and need backup in depth. Why?
Well, quite simply what happens when the file you need to restore has changed 4 times in 2 months? When did the file get damaged or corrupted? The first save? Second? Third? Fourth?
Please consider the fact that a 1 TB drive (USB, external) is $65-70 dollars USD. I'm not saying that you need a 1 TB drive, but that's not much.
For my recommendations, I typically suggest at least having enough storage for 30-60 days worth of backups. My backups at home with a 1TB drive use to last 6-7 months, until I started moving around 300 GB of data (organizing my downloads)....
For more information, and a related posting... Check here ("How Much Space should I reserve for Time Machine").