I'm a mac user running 10.4.11, and after the verizon update last week, my fascinate wouldn't mount as a data device. Here's what worked (from another forum):
1. plug phone into mac (debugging off)
2. choose "mount" from phone's drop-down usb menu
3. select "ignore" when mac says it can't recognize disc
4. in terminal, type:
gpt show /dev/disk1
5. hit enter, says it can't find drive
6. turn off usb storage from phone's drop-down
7. again, choose "mount" from phone's drop-down usb menu
8. in terminal, type (again):
gpt show /dev/disk1
9. Hit enter, and it found the drive, same as before. Probably a few too many steps, but it worked for me, so take it or leave it.