Q: can i use an USB cable to back up files to another mac from my laptop?

Hello DiB2,

Thank you for the question.  It sounds like you would like to transfer files between two computers.  You won’t be able to use a USB cable for this, but there are a few of other options to transfer the files. 

If both computers are Mac computers, you can use a Firewire or Thunderbolt cable transfer files between the computers.  You can use the steps in one of the following articles:

How to use and troubleshoot FireWire target disk mode

http://support.apple.com/kb/ht1661

OS X Mavericks: Transfer files between two computers using target disk mode

http://support.apple.com/kb/PH13842

Alternatively, you can enable File Sharing and transfer files using Wi-Fi or an Ethernet cable:

Mac Basics: File Sharing

http://support.apple.com/kb/HT1549

AirDrop is an additional option if both computers are running OS X Lion or later and are the same as, or newer than, these models:

• MacBook Pro (Late 2008 or newer)*
• MacBook Air (Late 2010 or newer)
• MacBook (Late 2008 or newer)*
• iMac (Early 2009 or newer)
• Mac Mini (Mid 2010 or newer)
• Mac Pro (Early 2009 with AirPort Extreme card, or Mid 2010)

Find more information about AirDrop here:

Mac Basics: AirDrop helps you share items with others nearby

http://support.apple.com/kb/ht4783

Thank you for using Apple Support Communities.

Best,

Sheila M.