Help with identification of diodes.

[Aglovale]

Hi there all. I'm a complete novice to electronics (although I am slowly learning basics) and I need some help with identifying these diodes.

The first image is a hard drive PCB. The purple diode has: QA RA13 code and is approx 4mmx2mm. The red diode has: 12T 09 code and is approx 4mmx2mm.
The second image is a mini PC. The orange diode has: J1 2 (2 is written sideways) and is approx 2.5mmx1mm.

My own research tells me that the hard drive diodes are tvs diodes therefore, I presume that the 12T would stand for 12v tvs? and I read that the QA on the second diode normally means 5v?.
As for the mini PC, I have no idea.

Can anyone help me identify these diodes so I can purchase replacements?
Thank you

(Images are in attachments)

[fzabkar]

QA is a 5V TVS diode.

12T is a 12V TVS diode.

TVS Diode FAQ:
http://www.hddoracle.com/viewtopic.php?f=100&t=86

J1 could be an N-ch MOSFET (BSS138LT1):
http://www.s-manuals.com/pdf/datasheet/b/s/bss138lt1_on.pdf

[Aglovale]

Thank you for your reply it was most insightful. I know nothing about mosfets so the link you posted with all that information is very helpful.

[Aglovale]

May i ask your help with 1 more smd.

Image 3 says: 30.000
                            109
                       There is also a symbol: a diamond shape with a bold T in the middle (I don't think you can see the code via the image unfortunately).
                       Approx 4.5mmx3mm

Is this a diode - does the 30 stand for 30v?

[fzabkar]

That's a 30MHz crystal.

[Aglovale]

That's a 30MHz crystal.

Thank you so much, I really appreciate your help.