Password: <root password goes here>
# mkdir /mnt/sd
# cd /mnt/sd
# mount /dev/sdb1 ./
That'll mount the SD card (/dev/sdb) to /mnt/sd, it might not be sdb for you. If you have a SATA HD, it most probably will be if you don't have any other mass storage devices in, otherwise if you have a different type of HD, it'll most likely be /dev/sda, providing you have no other mass storage in. Remember to specify /dev/sd*1, else it will complain about invalid block devices or something.
Before you rip it out, do: # umount /mnt/sd
If it says it's busy, but you're not copying anything, it's usually safe to take it out then.
You'll have to be root to put stuff on it. I suggest running something like # nautilus ./
(providing /mnt/sd is your working directory), then you can drag and drop files from other applications without permissions.
This is assuming you have a USB card reader.
If that fails, have a look at what syslogd says when you plug it in. If nothing happens (to do with hald or whatever), try a different port.
Oh, you're using the GP2X interface?
Hm, this should still work, as it's also USB, but I do suggest getting a card reader, they're really cheap (I got mine for £5), as they're a lot faster than the GP2X transfer.
You might have more luck on the Ubuntu forums - lotsa friendly people on there.
Describe it as a problem with a USB mass storage device not being detected. If you're lucky they'll help you to provide them with some diagnostic info. You might need to add support to your kernel if you've got some kind of weird motherboard.
The problem isn't your distro exactly - assuming you're using a relatively new version of Ubuntu - but more likely to be some strange hardware which isn't being detected properly.
// auto-merged, how weird:
Whoops - additionally you can connect anything to your ext port that you can connect to a breakout board, but you need to build a custom cable using a connector found on some mobile phone serial connectors - check the gp2x wiki for phone models known to work, bone up on your soldering skills and find a phone shop that sells the phone cable.
BOB might be more cost effective though, if you buy from an actual mobile phone accessory shop. Retail prices around here are like ~$40 au, plus whatever components you need. But I suspect that a home built cable wins if you're trying to make something portable.
Using the same ext connector you should be able to add JTAG for accessing the chipset, but I don't know anything about that stuff. Again, wiki it up.
then run it. this will update your gp2x. plug it in, turn on usb networking, run ifconfig usb0 up 192.168.0.1 or whatever ip address is relevant on your pc, and you should be able to communicate with gp2x
(yes, this is all ripped straight from this wiki page, but is helpful for google searchers, or if the wiki page is down)
now i'd done all this on my laptop, and gp2x was working fine, but i couldn't get it going on my desktop pc. turns out it was the hardware in the pc causing the issue. specifically, i have a motherboard with an nForce chipset, which i found out (by googling "ubuntu nforce usb") is not very good at detecting usb devices under current (2.6.20) linux kernels. i first suspected it was something beyond the gp2x when none of my usb card readers would detect over 1gb, even though i was using known working 2gb/4gb/8gb sd and cf cards
my solution in this case was to install a pci usb2 card with a different chipset (via in my case, though any would do), now all my card readers detect fine. still no gp2x tho, i'm guessing there are still kernel issues with that