I tried my question over at the Apple boards, and have gotten no response, so I’ll give the Dopers I shot–I know we have a few apple experts here.
First, about my Mac:
MacPro
OS X 10.6.8
2x2.66 GHz Dual-Core Intel Xeon
9 GB 667 MHz DDR2 FB-DIMM
4 internal hard drives, ranging from 1-2 TB each
Lately, my Mac’s USB connections have been wonky. I’ve tried the front, the back, using powered hubs, using no hubs, and this is generally what happens: Whenever I try to upload data to an external harddrive or stick, I seem to lose all USB devices except the hard drive itself. (And, sometimes, the stick gives me a problem and refuses to complete the transfer, unless I unplug all USB devices immediately after initiating the transfer.) I’ve tried what in theory should be a powered hub for my other USB devices, but that doesn’t seem to have any effect on the problem. I tried resetting the PRAM, to no avail.
So far as I can tell, the problem does not occur when I pull files FROM a USB device. Nothing goes haywire if I’m transferring from an external to an internal, only vice versa. When I lose my USB devices, I can remount them by unplugging and replugging the hub (or individual USB connection) in, but (once again, so far as I can tell) only when the data transfer to the external USB drive is complete. If my computer goes to sleep before the data transfer is complete, I usually have to wake it up from a screen sharing connection on my laptop, and then I can unplug and replug to get everything working again.
I apologize that my symptoms are peppered with a lot of "usually"s and “seems.” The only thing that I can tell is consistent is the connection going when I upload to an external USB drive. And it doesn’t go immediately. It takes about a minute or so before I lose the connectivity of other USB devices (keyboard, mouse, graphics pad.)
The severity of the problem is recent (within the last three months.) Before, I would occasionally get an external drive going offline and remounting, but I assumed that was just a flaky connection. The problems occur no matter whether I use the front or the rear ports.