Can I retrieve erased data after an update to Snow Leopard

If the hard disk was erased and you don't have a backup then you are fairly unlikely to be able to retrieve the data. There are a number of retrieval programs, but they generally don't work all that well. A professional service would stand a higher chance but this would be extremely expensive.

Either way, the more you go on using your Mac the more likely it is that you will over-write any files which may still be there so if you want to try retrieval you should stop using it immediately.

Data Recovery - Recovering Deleted Files

Data Recovery - Recovering Deleted Files (2)

Stop using the computer until you decide whether or not to try data recovery software.

Di you erase the drive before installing Snow Leopard ? - it's not normal for files & data to be deleted unless that happens.

Sometimes, people report everything being missing, when there's actually some confusion over the User accounts, and the one currently logged in isn't the same one that was previously used...

Are you the only user shown in System Preferences - Accounts; and the only one named in Finder - HD/Users ? (other than shared).

Do you by chance have an iPhone/iPod? Were your pictures on their as well? If so, you may be able to retrieve from them.

...and since no one else has said it, ALWAYS back up before doing any type of a major upgrade. In fact, backup frequently just to do it.

I am not sure how much the software packages cost, but maybe if you contact the vendor diectly and explain your situation they will give you a break on the cost. (It cost nothing to ask.)

Data Rescue, from Prosoft Engineering, is an fairly advanced package...it's helped me recover stuff several times (works better with smaller files, less successful with video files). It's currently retailing for $99. Whenever I buy a new computer, I always purchase a copy of this program, as I've had the disc crash blues more times than I care to admit...