Sourcing components & personal experience
I thought I would chip in with my positive personal experience of installing a second drive into a non-server 2011 Mac Mini.
The job is undoubtedly fiddly, and must be approached with due care and attention, and the patience of Job. But it is not difficult. The iFixit teardown photos are a very useful guide to the exercise, and the procedure outlined by iFixit should be followed to the letter.
Being based in the UK, I sourced the SATA adapter from
www.applemacparts.co.uk (VIS Ltd), and paid £25 + VAT +postage for part #076-1391. The other commonly referenced cable is identical, though for some inexplicable reason VIS recommend 076-1391. It took the best part of two weeks for the cable to arrive through the post.
Information on screws and grommets was harder to come by, but I soon learned that any 6.3mm (1/4 inch) M3 screws would do, though possibly with a little filing of the heads. From Maplin I bought a packet of 50 for a couple of quid. The code number for the screws is "A13BJ". For the grommets, I bought (also from Maplin) a small packet of "Open grommets", the code number for which is "QT92A".
There is no need to purchase the iFixit kit at great experience, and possibly incur import taxes if bought from outside the US. If using the above-referenced screws, two of them will need to have their heads filed down a little. These are the ones that will sit against the outer wall of the Mac Mini case. File away as much as possible, while leaving enough of a cross so that the screwdriver will still engage.
The installation went without a hitch, and the result is a 2.5GHz (Radeon) Mac Mini with a 160GB Intel SSD as the primary drive, and a 750GB, 7200rpm WD Scorpio Black as the secondary, used for Time Machine backups and general storage.
However, after all this, I've decided that the Mac Mini is not for me, and will therefore sell it. I've yet to decide whether to advertise it as a modified machine (though without the SSD, and with the original 5400rpm HD back in place), or strip out the extra drive and SATA cable, and sell these separately. This post is not a for-sale advert, but you are welcome to PM me and discuss the matter. I am based in London, UK.