I have an unknown error (1621) and my iPod is not being recognised in iTunes or the Finder, even in Disc mode. Anyone know what's going on?

Try and force iPod into Recovery Mode:

1. Turn off iPad

2. Connect USB cable to computer; leave the other end alone

3. Press and hold the Home button down and connect the docking end of cable to iPad

4. Continue holding the Home button until you see the "Connect To iTune" screen

5. Release the Home button

6. Open iTune

7. You should see "iTunes has detected an iPad in recovery mode"

8. Use iTune to restore iPod

Note: You need to be patient and repeat the above many times to recover your iPad

Try a new cable

Try DFU mode and then restore

How to put iPod touch / iPhone into DFU mode « Karthik's scribblings

Try another computer