Migration Assistant Issue - Hard Drive Memory Related

Question

I've been trying several days to migrate my system from a 2009 iMac to a new MacMini with no success.  I've tried Migration Assistant over WiFi, Ethernet to my AirPort images and directly in Target Disk mode (Firewire 800).  Both machines are current and running 10.10.1.
When I first launched Migration Assistant on both machines the old iMac never sees the MacMini although the MacMini sees the iMac and generates the six-digit code.
I next tried connecting to the Time Machine backup of the iMac and encountered the persistent fault that I can't seem to solve.  I can find the server and files but once I get to the screen where it asks what files to Migrate it always calculates not enough space to move the files.  The MacMini has 846 GB available and I'd like to move 338 GB over from the iMac.  I have no clue what is causing this.
When I deselect items such as Photos and Music nothing changes, in fact I can deselect all but 100 MB and it still says there's not enough Hard Drive space on the MacMini.  I've made sure that there isn't a same user name and run Disk Utility Disk Permissions many times to ensure the hardware is good.
I manually moved the Music and Pictures and reduced the Time Machine image size to under 250 GB but I still get the same message each and every time.  After spending $30 today on a Firewire 800 cable I'm at my wits end.  Can anyone suggest where to go next? I've looked at the console log and don't see anything that can point me in the next direction...
Should I get on the console and call the Setup Assistant and erase my machine after I've taken hours to transfer the Pics and Music or is there a better way?

Answer 1

What data do you actually want to migrate? Is it just 'user data' or applications & other files?
For a user data only migration
If the iMac is still setup with the user account I would forget about the Time Machine disk or using Migration Assistant and try a manual migration. It is an involved process, but if the files copy OK you can create the user account & it should be fine.
Start the old iMac in a normal boot, login as an admin user, ideally in an account you do not want to transfer to the Mini (create a new one if you need).
Boot the Mini in Target Disk Mode and attach to the iMac.
You need to copy the desired user home folder(s) from the iMac into the /Users folder on the Mini. 
Assuming this file copy works (it's unclear if the issues in MA are due to problems in your file systems) you can disconnect the Mini & reboot it.
     If the copy fails use recovery mode to repair both Macs - repair the filesystem & permissions in Disk Utility & retry.
On the Mini open 'System Preferences > Users & groups' and authenticate. Double check the name of the user folder you copied over, this is the users 'short name' a.k.a 'Account name' it must not contain any spaces, the 'Full name' can have spaces.
Create a new user with the 'foldername' as the 'Account name'. If it is correct you will be prompted to 'use existing folder'.
Agree to use that folder & the OS will make the new user & assign the correct permissions to that home folder. If you get no prompt double check the name matches the folder in /Users.
Repeat for any other user accounts.
Reboot the Mini & try to log in as the new user.
I would not attempt to migrate applications via this method - too many files can be scattered all over the disk by Application installers, it's simpler to reinstall using the installers, grab new copies from the developer or from the App Store etc.
Network & System settings will also be left behind & iTunes will require you to authorise the Mac, but most other things should be fine inside the migrated user account.
If you are uncertain ask & I'll try to clarify, as ever backup important data before you begin, disconnect the backups to reduce the risk of accidentally deleting the wrong thing & good luck