A number of Snow Leopard users have run into a problem where upon backing up with Time Machine they get an error "109" claiming the backup disk image could not be accessed. This is happening for people who are backing up via the network, and mainly when using an Airport Extreme Base Station with an attached hard drive for their backups.
This problem started for people after upgrading to Snow Leopard, and a number who still have machines with 10.5 Leopard installed have found those to continue backing up perfectly fine while the Snow Leopard machines are having troubles. This indicates the problem is a compatibility issue between Time Machine in Snow Leopard and the backup hardware setup.
Specifically, users are able to complete the initial backup of their computers, and afterward the backups run fine; however, anywhere after a day to a week of steady backups, Time Machine will stop working and display the error.
AEBS not compatible
Many people seem to be under the impression that the Airport Extreme Base Station is a perfect setup for Time Machine, given it provides a network drive that the Mac can see and recognize in Time Machine. Additionally, a number of people are being told by Apple sales representatives that the AEBS is a solution that will work. Unfortunately this is not the case, and Apple has released a few knowledgebase documents such as the following which outline the incompatibility between Time Machine and the Base Station:
While the AEBS may work, it is not supported by Apple as a Time Machine solution. For this, Apple has released the Time Capsule. Despite this, since the Time Capsule and Base Station are essentially the same device and have worked similarly under Leopard, people have tried a variety of approaches to getting their AEBS working again. The only ones that appear promising are the following:
Rename computer and drive
Rename the computer, Airport station, and Time Machine drive to a short and simple name with no punctuation, spaces, or special characters in the name (ie, "Bob" vs. "Bob's computer").
Create a new network location
In the "Network" system preferences, create a new network location and add only the network interface you use to connect to the Time Machine device.
Turn off backups when on battery power
For laptop users, In the Time Machine system preferences click "options" and uncheck the option to backup when on battery power, and then only back up when connected to your charger.
Time Capsule users affected
If you use an Airport Extreme Base Station you are using an unsupported method of backing up via Time Machine; however, a few people are having this same problem with their Time Capsule devices. The Time Capsule and AEBS are built almost identically, and there are only firmware differences that define their alternate functions and limitations.
If you have problems with your Time Capsule, the only suggestions I have at this point are those I listed above. Reducing the number of points where network interruptions can interfere with the connection to the Time Capsule may help the situation.
Overall, this problem does appear to be an issue with Time Machine in Snow Leopard, and while these suggestions may reduce the prevalence of the problem, a true fix will probably have to come from Apple. Unfortunately, they will not spend time addressing problems with Time Machine on Airport Base Stations; however, if you have these issues with a Time Capsule, contact them about the situation so they can get a better sense of what is going on. This can be done in the following ways: