been doing the same as this for years, I constantly have problems once I insert the usb disk that contains my keychain, that if i attempt to eject it safely, it says i have to quit effectively every program on the computer before it will alow me to eject it ? and sometimes it even wants me to quit the finder to eject it, so I end up force ejecting the usb disk, the problem is the dialog that comes up and says I have to quit programs before I can eject the usb disk. Gives no indicator as to which keychain item is causing this issue, I have 400-500 keychain items so its a drag to establish where the problem lies, ever experienced the same … any ideas ?

Ok I was attempting to do a target disk mode install of snow leopard from a thunderbolt 2011 imac to a a slighty earlier manufactured unibody macbook pro whose restore disk was 10.6.3, but i was using a usb flash disc clone of an SL vanilla 10.6.0 install disk on a 2011 imac whose restore disks are 10.6.6 only, ok as listed below when the install entered its second reboot phase , why it does a reboot now i dont know but its irritating to say the least especially when your installing on an external volume and not the boot volume ? at this point my imac proceeded to spool shell text and then kernal panic and refuse to boot ! bear in mind here … I wasnt even attempting to install on the imacs internal disk at all, just via it to a macbook pros hard drive in target mode connected via firewire to the imac, this boot failure on the imac did not resolve itself before I had smc reset the imac removed all peripherals done disk and permissions repair on the imacs disk !, probably the smc reset was the crucial action. Ok the moral of this story is this if installing snow leopard via target mode to an external disk/machine, make sure the version of snow leopard your installing is a version capable of booting the host machine which is doing the installing and capable of booting the machines whose drive your installing to via target mode or ideally the newest version of the installer you can get ie 10.6.8 otherwise at the reboot phase of the install the host machine running the installer may well fail or on restart of the target disk installed machine it will also fail. Not all snow leopard install disks are created equal. at least make sure the version of the installer your running is equal or later in version than the restore discs of all machines involved regardless of the age of machine whose drive you are installing to. Or of course doing a disc/dvd based install you could try the dvd swap trick listed below :

this also indicates that if you are buying a retail snow leopard install disc you really need to make sure its a 10.6.8 retail install disk, otherwise it will fail to work on half the machines you want to install it to ! nearly all early snow leopard retail disk will sadly be 10.6.0 and will fail to boot loads of the machines released after the retail disk release ! you are warned. would apple tell you of any of this complexity, you must be joking apple dont give a shit about informing the user of this kind of mess!

please note i am repeating the information below from a post by john fair and all credit goes to him, wont really work for a usb flash disc based install of OS X but there we go, dvd installs this will solve your problem, by disk swapping at the right moment.
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After 6 days and NUMEROUS attempts I have figured out how to get Snow Leopard onto the new Mac Mini!! It turns out I wasn’t thinking the problem all the way through. I thought that the Mini was causing the kernel panic when connected in Target Disk Mode. However I was trying to install the retail Snow Leopard (which is 10.6.0) onto the Mini using an MBP which required 10.6.3!

There is a step after all of the applicable files are copied to the Mini (TDM) where the Source machine reboots and is supposed to continue the install. This is where my MBP was freezing up and I couldn’t understand why. Well it turns out that the MBP is attempting to boot from the SL disk (10.6.0), however this machine requires a minimum of OS X 10.6.3 so the boot was hanging.

I ejected the retail version of OS X, input the Recovery disk I got with the MBP (which is 10.6.3) and the install continued as usual. Once installed I booted directly to the Mini’s hard drive (TDM) via the MBP and ran Software Update to get to 10.6.8. Then rebooted and let the Mini boot itself…and we’re up!

Steps:
1) Put Mini in Target Disk Mode (Firewire only)
2) Put SL retail install disk into Host Mac and install to the TDM drive
3) When the reboot hangs power the machine down (or if you’re fast hold down Option on that reboot) and restart holding Option.
4) When the boot choice screen appears eject the Retail disk and input your Recovery disk.
5) Boot to the Recovery disk and the install picks up automatically (don’t worry, it’s actually installing 10.6.0 even though you’re booted into 10.6.3’s recovery).
6) Once you’ve gone through the new user introduction stuff (input username, etc) run Software Update
7) Power down the Mini and restart it w/o TDM.
8) Profit