no mountable file systems (again!)

I know that this problem has been discussed before, but that was for older Macs going back 2-3 years ago. Well, over the past few months I keep getting this error message and my .dmg files don't open after downloading (not all of them but quite a few). I have a PowerPC G5, 4 x 2.5GHz, running the latest OS X (10.4.10). Any help will be appreciated!

Can you mount them manually? Disk Utility can also mount dmg's.

Funny you should say this, my original account became corrupted so I am now using alternative ones. And for some reason the dmg files are not automatically mounting. I can't find anything in prefs in system, finder, disk copy or browser to control this. So I'm a little confounded too. It only happened after I reinstalled OSX, then set up new accounts & now also use one of the earlier test accounts.

When I try to mount them manually using Disk Copy, I get the following error message: <file name>.dmg failed to mount due to error 95.(EMULTIHOP (Reserved))