IMac not recognising external drives or memory cards

Question

Hi,
I've been using both a Lacie external drive and a standard SD card from transferring files and images on my computer for ages, with no problems. I have two issues, possibly related, possibly not.
1) I accidentally removed my Lacie drive before it had properly ejected the other day. The message that I had done so popped up on the screen so I plugged it back in, (as the Mac will usually fix this when it happens) and it never opened up in my finder. Tried restarting machine, but it still cannot even see the drive. It's an old external drive, and was backed up, so I put the problem down to that and almost forgot about it.
2) Today I plugged in the SD card from my camera to transfer some images. After the transfer, I ejected the card correctly, then reinserted it to do a last job. Again, nothing in the finder. It's like neither of the devices are even there.
I've checked finder preferences to make sure that I can see external hard drives - this is on.
I've done the old fashioned restart too. No luck either way.
An additional point is that my BACKUP drive IS visible in finder.
Any ideas?
I'm worried now that I shouldn't plug in any external drives, especially my work ones, just in case there is something seriously wrong.

Answer 1

Please read this whole message before doing anything.
This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.
The purpose of this exercise is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login. Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode and log in to the account with the problem. The instructions provided by Apple are as follows:
Be sure your Mac is shut down.
Press the power button.
Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
Note: If FileVault is enabled under Mac OS X 10.7 or later, you can’t boot in safe mode.
Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs.
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. Same problem(s)?
After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of the test.