user17310: When I plug in my nanoatmega328 to my computer, nothing shows up in /dev as I expect it to. If I plug in my Uno, /dev/ttyACM0 shows up. I have tried several Nanos.
Linux Mint 17.2.

user7856: Did you try with lsusb and dmesg?
They should show if the Arduino was detected and what are its parameters (use lsusb -v for more detailed info).
If it is detected but you do not get the corresponding tty device, it might be because of missing/different rules.
You could check out this link and edit the parameters idProduct and idVendor to match what you get from lsusb, in case your rule is not matching.
Then remember to reload the modified rules.
Protip: compare the parameters of your working Uno against the ones from the Nanos and the values set in the rules.

user4143: dmesg should be the tool to help you.
Run:
$ dmesg -w

Then hit return a few times (to get some blank lines). 
Next plug in your Nano.  Some text should appear on the screen, which will give you a clue as to what is going on.  I don't have a Nano to try with, but a typical working device would look something like:
[29125.599800] usb 9-1.4.1: new full-speed USB device number 13 using ehci-pci
[29125.694742] usb 9-1.4.1: New USB device found, idVendor=04d8, idProduct=0208
[29125.694747] usb 9-1.4.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[29125.694749] usb 9-1.4.1: Product: Lenny
[29125.694751] usb 9-1.4.1: Manufacturer: chipKIT
[29125.694753] usb 9-1.4.1: SerialNumber: CK1660A05353C5
[29125.695213] cdc_acm 9-1.4.1:1.0: This device cannot do calls on its own. It is not a modem.
[29125.695240] cdc_acm 9-1.4.1:1.0: ttyACM0: USB ACM device
[29125.695768] cdc_acm 9-1.4.1:1.2: This device cannot do calls on its own. It is not a modem.
[29125.695791] cdc_acm 9-1.4.1:1.2: ttyACM1: USB ACM device
[29125.697772] input: chipKIT Lenny as /devices/pci0000:00/0000:00:1a.7/usb9/9-1/9-1.4/9-1.4.1/9-1.4.1:1.4/0003:04D8:0208.0004/input/input19
[29125.698394] hid-generic 0003:04D8:0208.0004: input,hidraw2: USB HID v1.11 Mouse [chipKIT Lenny] on usb-0000:00:1a.7-1.4.1/input4

If it doesn't know what the device is it will give you a fraction of the information, and probably tell you it can't attach a driver.
If the device is faulty and not recognisable you will get messages similar to:
[29125.698394] usb 2-2.1: device not accepting address 5, error -71

If you get absolutely nothing displayed then it can't even recognise that anything has been attached to your computer.  Try different USB sockets and different USB cables to see if that helps.  
Note: some cheap USB cables used purely for charging small batteries don't have any data wires in them - just power. Using one of these will fail for sure.

17310: <p>When I plug in my nanoatmega328 to my computer, nothing shows up in <code>/dev</code> as I expect it to. If I plug in my Uno, <code>/dev/ttyACM0</code> shows up. I have tried several Nanos.</p>

<p>Linux Mint 17.2.</p>
 7856: <p>Did you try with <code>lsusb</code> and <code>dmesg</code>?</p>

<p>They should show if the Arduino was detected and what are its parameters (use <code>lsusb -v</code> for more detailed info).</p>

<p>If it is detected but you do not get the corresponding tty device, it might be because of missing/different rules.</p>

<p>You could check out <a href="http://playground.arduino.cc/Linux/Udev" rel="nofollow noreferrer">this link</a> and edit the parameters idProduct and idVendor to match what you get from <code>lsusb</code>, in case your rule is not matching.</p>

<p>Then remember to <a href="https://unix.stackexchange.com/questions/39370/how-to-reload-udev-rules-without-reboot">reload the modified rules</a>.</p>

<p>Protip: compare the parameters of your working Uno against the ones from the Nanos and the values set in the rules.</p>
 4143: <p><code>dmesg</code> should be the tool to help you.</p>

<p>Run:</p>

<pre><code>$ dmesg -w
</code></pre>

<p>Then hit return a few times (to get some blank lines). </p>

<p>Next plug in your Nano.  Some text should appear on the screen, which will give you a clue as to what is going on.  I don't have a Nano to try with, but a typical working device would look something like:</p>

<pre><code>[29125.599800] usb 9-1.4.1: new full-speed USB device number 13 using ehci-pci
[29125.694742] usb 9-1.4.1: New USB device found, idVendor=04d8, idProduct=0208
[29125.694747] usb 9-1.4.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[29125.694749] usb 9-1.4.1: Product: Lenny
[29125.694751] usb 9-1.4.1: Manufacturer: chipKIT
[29125.694753] usb 9-1.4.1: SerialNumber: CK1660A05353C5
[29125.695213] cdc_acm 9-1.4.1:1.0: This device cannot do calls on its own. It is not a modem.
[29125.695240] cdc_acm 9-1.4.1:1.0: ttyACM0: USB ACM device
[29125.695768] cdc_acm 9-1.4.1:1.2: This device cannot do calls on its own. It is not a modem.
[29125.695791] cdc_acm 9-1.4.1:1.2: ttyACM1: USB ACM device
[29125.697772] input: chipKIT Lenny as /devices/pci0000:00/0000:00:1a.7/usb9/9-1/9-1.4/9-1.4.1/9-1.4.1:1.4/0003:04D8:0208.0004/input/input19
[29125.698394] hid-generic 0003:04D8:0208.0004: input,hidraw2: USB HID v1.11 Mouse [chipKIT Lenny] on usb-0000:00:1a.7-1.4.1/input4
</code></pre>

<p>If it doesn't know what the device is it will give you a fraction of the information, and probably tell you it can't attach a driver.</p>

<p>If the device is faulty and not recognisable you will get messages similar to:</p>

<pre><code>[29125.698394] usb 2-2.1: device not accepting address 5, error -71
</code></pre>

<p>If you get absolutely nothing displayed then it can't even recognise that anything has been attached to your computer.  Try different USB sockets and different USB cables to see if that helps.  </p>

<p>Note: some cheap USB cables used purely for charging small batteries don't have any data wires in them - just power. Using one of these will fail for sure.</p>
