I've not used any Thunderbolt Docks but posting for feedback from other Thunderbolt Dock/USB3 drive owners.
"Just giving you my experience and seeking answers to a few points I have concerning thunderbolt docks. I have two issues with various thunderbolt docks:
1- booting from an external drive with USB3 connection to a thunderbolt dock does not boot the external which has a known good OS X install. I usually try to boot either Mavericks or Yosemite.
2- installing OS X, usually I do Yosemite, does not reboot the external drive again connected by USB3 to a thunderbolt dock. So in effect the second phase of the install will never complete.
I've tried mostly bus powered LaCie rugged drives. I have tried powered LaCie external USB3 drives.
I've tried the Belkin TB and TB2 Dock without success. I have currently also tried the CalDigit TB2 Dock without success. Their FAQs state it should work on USB3 Macs specifically 2013 or later. I've tried on an early 2011 MBPro, mid 2012 rMBPro, early 2013 rMBPro and a mid 2014 rMBPro. None would take the external connected to the Caldigit TB2 Dock and boot OS X. (Not tried them but didn't see boot support mentioned in product pages for Elgato TB1 or OWC TB2 docks.)
I do have friends who can boot from the Belkin Dock and even the first generation Caldigit Dock but it was on late 2013 rMBPro. (his reply below)
If any readers with a Thunderbolt dock have tried the above, a note if it's worked or not. Please include info on TB Dock, Mac model, OS X version (on Mac and USB drive) and info on USB3 drive used. Thanks. (Makes me wonder after a boot/restart (bus reset?) if the USB drive is ready fast enough. Or a bridge/chipset issue?)
Replies to above post: (later added first)
(Reply to question I'd asked OWC about their Thunderbolt 2 dock)
This is Chris from Other World Computing. We have performed some testing and found that Thunderbolt 2 Docks are bootable over USB 3.0 if:
1) The Mac is a Mid 2013 or later model
2) The OS being booted to is OS X 10.9.4 or later.
Other World Computing, Inc."
Re: TB Docks and USB3 drives
Greetings from Italy. Yesterday I was happy to receive my new Thunderbolt Dock 2 from OWC, that I bought in pre-release last month. I connected to my 'old' iMac 21.5" Late 2011 (TB1 port) with Mac OS X Yosemite 10.10.2 and I was happy to see my external USB3 drives (WD Passport 1TB and WD Elements 2TB) had fast performances, testing them with Intech Software QuickBench 4.0 (90MB/sec read and 85MB/sec write), surely faster than my FW800 WD 500 (50MB/sec read and 42MB write). (See below for his reply and later USB3 benchmarks.)
Then I tested all 3 HDs on booting them: selecting the USB3 drives from System Preferences and restarting or choosing them in the menu of drives with option key held on boot, none booted. Firewire drive booted in both cases. (Both methods.)
Thanks Guido. Some commented on your 2 WD USB3 drives performance. Both are portable drives (2.5in) and 5400rpm I'd guess? If so, even with a direct PC USB 3.0 connection they may not do much better, even empty. (Not a WD Passport HD but example USB3.0 1TB 5400rpm/8mb cache 2.5" QB scores (cache enabled) direct connected shows large file Avg of 101.9MB/sec Read, 100.1MB/sec Write (empty). And here's Guido's Dock Connected USB3 WD 1TB (713GB free) extended tests w/cache enabled for comparison - shows large file avg of 107.8MB/sec Read and 114.8MB/sec Writes.) Since your iMac doesn't have USB3 ports, not possible to compare a direct connection vs dock but if you ever test them with a direct USB3.0 connection, send the QB rates for comparison. Thanks.
Update: Guido replied with more info and later benchmarks on his WD portable USB3 drives:
"...about the (USB3 HDD) tests I did with my new OWC ThunderBolt 2 Dock.
Yes, both USB3 HDs, WD Passport 1TB and WD Elements 2TB, are 2.5in and 5400rpm, bought in 2014, and was formatted after connecting them to the dock, a few days ago. WD Elements 2TB is empty (no files on it) and WD Passport 1TB is full at 25%.
I tested them again this morning with QuickBench 4.0.7, doing an extended test and a standard test with and without cache and I'm now sending you the results. (images below are standard tests with/without Cache Effects.)
Unfortunately, at home and at office I only have old iMacs with USB2 ports, so I was unable to test them with a direct USB3 connection. This is also the main reason I bought the OWC TB2 : to have a fast access to my new drives.
best wishes, Guido"
"Boot from USB3 via TB Dock
I use a recent Startech TB2DOCK4KDHC dock and have had no issues booting my early 2014 and early 2013 rMBP OS X 10.9.5. (Using USB3 drive?) This dock seems to work much better than others on the market, like how they fixed the audio pop on sleep/wake and the eSata port. I used the TB1 version for near a year and had none of the issues reported by users of other docks. Its been solid for me, but I didn't try booting from a USB drive. Booting from drives in a TB enclosure was not a problem for me the few times I found the need.
So you have booted from a USB3 drive connected to the TB2 model dock? (Assuming the later comments are referring to the TB1 dock not tested for USB3 drive boot.)
"I have a MacBook Pro Retina 13" (Mid 2014) and an Elgato Thunderbolt Dock (the old one, not the new Thunderbolt 2 one). I have a Samsung 840 EVO (500 GB) in an external case (ICY BOX) an am able to start from the SSD when it is attached to the dock.
Best regards, Thomas"
(added 2/6/2015) "Booting from a USB3 drive connected to a Thunderbolt dock
I have a Caldigit Thunderbolt dock (original model not current TB2 version) and a late 2013 rMBP.
I have successfully booted from a number of different drives and SSDs connected to the USB3 ports of the Caldigit, including WD passport 2TB, Sandisk and Crucial SSDs on a Seagate USB3 Go-Flex adapter, Transcend Storejet enclosure, Sandisk Extreme Pro USB3 memory stick.
PS: I am the one "DH" mentions in original post that can boot a late 2013 rMBP"
Are you close enough to "DH" that he could try his MBP models with your TB Dock/USB3 drives?
(added 2/6/2015) "re: Problems booting from USB3 drives connected to Thunderbolt Docks
Did NOT boot. Did NOT appear in menu of drives with option key held on boot.
Tried several of the USB3 ports on the dock.
Boots fine Thunderbolt Pass-Thru and USB 2 ports on Macbook
Macbook Pro Early 2011
OWC Thunderbolt 2 dock (just got it) Not really using it as still on Mt. Lion
(OWC ThunderBolt 2 Dock page lists OS X 10.9.4 or later.)
LaCie Rugged USB3/Thunderbolt case with SSD (Yosemite).
(Had a reader ask I post a reminder about OS X 10.10.2 Thunderbolt problems (kernel panics if ejected TB drives powered off, etc). Sample forum threads on the problem here, here, here and here. Some have reverted back to OS X 10.10.1 until a fix is released.)