Currently, postings to an Embassy ARE on a voluntary basis, it's hardly a good security practice to force the keeper of the keys to go someplace they don't want to be. While it is technically possible for someone to be ordered to serve at an Embassy (as with any other posting) given the normal complement at an Embassy is 1-2 pers, you hardly want a "disgruntled employee" to be the one overseeing security. Additionally, Ambassadors have final say about who serves at their Embassy and I highly doubt any of them would be too keen to accept someone who was being ordered to attend the post. Positions stay vacant because no-one suitable is available.
As for being eligible to go on completion of your PEP. While this is also technically true, in reality this might get you to Kabul but no-where else. The CM is not going to post you to an Embassy only to have to pay to bring you back for your DP2 when there a huge pool of interested people who already have the course.
Posting length - As pointed out, a few Embassies are 1-year postings. All other Embassies are 2-year postings with the possibility of 2 x 1-year extensions to a max of 4 years. Just because it is a 2-year posting does not mean you will remain for 2 years no matter what. If it becomes apparent that you need to be repatriated due to family problems, insistence on leading a high risk lifestyle etc, you will be sent back no matter how long you have been at post. The current record for minimum time on post is less than a month…although recently one guy "quit" between the time his HG&E shipped and when he arrived at the Embassy.
MSGU is in the process of expanding by 25 positions over a 5-year period with a mixture of new posts being added and old posts being augmented, so the odds are improving slightly for Jr NCOs.
Important things to note at the Cpl/MCpl rank:
The only time you can volunteer for a specific Embassy is if it is a high risk Mission, at this point that means Kabul.
For the other missions, the CM selects a list of eligible people who have indicated their desire for MSGU via their posting preferences. People from that list are directed to screen for a posting to MSGU with MSGU HQ deciding which Embassy you will go to via your Résumé and, perhaps, a telephone interview. You do not get to choose which Embassy you will go to, if you don’t like where MSGU HQ is sending you, your only choice is to withdraw your name.
Just because you pass the screening for an Embassy does not mean you will go to an Embassy.
Competition is fierce at the Cpl/MCpl rank due to the number of available positions. If you do want to go, ensure it is #1 on your posting prefs every year. The first question I always ask someone who complains they have never been to an Embassy is what their posting prefs are. 99% of the time it’s 3 inside Canada locations.
As an added “bonus”, there are 1 x Cpl and 1 x MCpl position at MSGU HQ (plus 1 x Maj and 1 x MWO) in Ottawa so it is possible to be posted to MSGU without being posted to an Embassy. The upside is you'd probably do TD at a variety of Embassies, the downside is you wouldn't qualify for most of the allowances or the Foreign Service Premium points while on TD.
Having “been there and done that”, if you have any specific questions, feel free to ask.
Watchdog-81: In 20+ years, I’m aware of 2 x Secur O/MPO who have gone CFA/CDA, both Cols, you know of any others?