I have all four of our laptop Macs backing up to a single 3 TB hard drive that is connected to the new 802.11ac Airport Extreme Base Station. The hard drive is partitioned into 4 volumes, one for each of the laptops. It has been working perfectly fine for the last several months, but today one of the computers, an i7 Sandy Bridge MacBook Pro, received this error message: Time Machine completed a verification of your backups on AirPort Extreme. To improve reliability, Time Machine must create a new backup for you Click Start New Backup to create a new backup. This will remove your existing backup history. This could take several hours. Click Back Up Later to be reminded tomorrow. Time Machine wont perform backups during this time. I'm prompted with two buttons, one to erase the sparse image and start over with a new backup, and one to remind me later (dismiss). A strange feature is this sparse image is locked (pad-locked icon) but none of the other computers' sparse image icon is locked. I unlocked it with Get Info but that does not fix the problem. The sparse image does not show up in Disk Utility, even though I have opened it in a finder window. None of the other 3 laptops are having this problem. All four are running 10.8.4 Mountain Lion. Any ideas how to get this fixed without redoing a backup?