Sparse bundles created by Time Machine in the latest versions of Snow Leopard are created slightly differently than they used to be. It used to be that Time Machine would create a sparse bundle with a name like “machine-name_001122334455.sparsebundle” where the “001122334455” part was your main ethernet port’s MAC address. Now it creates just “machine-name.sparsebundle”. So how does it associate a machine with the sparsebundle?
Well, it turns out they added a new file inside the bundle. Now, alongside “Info.plist” there is a new file called “com.apple.TimeMachine.MachineID.plist”. Inside this file is some info:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>VerificationDate</key>
<date>2011-04-09T19:57:48Z</date>
<key>VerificationExtendedSkip</key>
<false/>
<key>VerificationState</key>
<integer>1</integer>
<key>com.apple.backupd.BackupMachineAddress</key>
<string>00:11:22:33:44:55</string>
<key>com.apple.backupd.HostUUID</key>
<string>01234567-1234-5678-9abc-12345678abcd</string>
</dict>
</plist>
The “com.apple.backupd.BackupMachineAddress” is where the MAC address is now stored, but there’s one other extra field: “com.apple.backupd.HostUUID”. This value can be found by launching “System Profiler” (hold down the option key while selecting the Apple menu to get there quickly). On the first page (titled “Hardware Overview”) is something called “Hardware UUID”. This is what goes in the HostUUID field.
So if you ever need to create a Time Machine sparse bundle from scratch, you’ll need to make this file and fill out those 2 fields. There are some other fields in there but I have no idea what they do. I think they have to do with when “fsck” was last run on the sparse bundle and whether or not the bundle is valid, but I haven’t explored it yet.