First thing I would check is the drive connections...make sure cables to drive from both power supply and motherboard are tight, not loose.
For external drives...just cut the power or disconnect from USB port.
The error message specifies an "IDE" drive as a problem...I can't tell if your external is an PATA or a SATA drive...but it seems that Windows will often balk if a USB device is detected during boot (other than keyboard/mouse) because it's in an "on" state. The cure for that is to turn the drive off or don't connect the external drive until the system boots properly.
I see that your drive doesn't seem to have an On/Off control, if I'm reading the WD description correctly. "Turns on and off with your computer. Enters idle mode when not in use."
Which says to me that Windows detects it and cannot understand why it's attached because the USB drivers haven't been loaded yet.
I would start with booting minus the external drive being attached and in an ON state.
As suggested previously...take it one step at a time. Dsconnect your second hard drive also, leaving only the optical drive and the drive containing Windows properly connected.
Then...try to boot system normally. If error message, write it down and post.
<<What's the best way of checking whether they're sata and/or ide/ata drives?>>
The best way, IMO, is to just take a look at them...in the case of an external drive, look at the documentation for same.http://www.wdc.com/en/products/products.asp?driveid=760
Do those things and we'll know if anything else is necessary.
<<with the internal drive, do I just unplug the power cable before booting up?>>
Unplugging just the PSU cable alone is not what I would do. It's easy enough to unplug both cables to a hard drive connected to the motherboard.
Edited by hamluis, 20 April 2010 - 01:26 PM.