You looking to just transfer to your internal SD and external? Or just one?
I haven't tried it with my mac...but my PC wouldn't recognize it, even with updated drivers...
I put it into Debug mode(it is ALWAYS on when plugged into a usb port, so I can use my phone). Once it is plugged in, pull down the notification bar, You will see USB debugging mode, and underneath that, USB something or other...ha. Press that, and tell it to mount....works like a charm on my PC.
If worse comes to worse, for your external SD card...just use a card reader to transfer files. Sure it involves taking the card out of the phone...but can't fail.
For internal, if you only use it once in a while for transfers, roms and whatnot...I would send the files through email and get the files that way...
Hope something I said helps.