10.10.4 will not install

Try these in order testing your system after each to see if it's back to normal:

1. a. Resetting your Mac's PRAM and NVRAM

b. Intel-based Macs: Resetting the System Management Controller (SMC)

2. Restart the computer in Safe Mode, then restart again, normally. If this doesn't help, then:

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the

COMMAND and R keys until the Utilities menu screen appears. Alternatively, restart the

computer and after the chime press and hold down the OPTION key until the boot manager

screen appears. Select the Recovery HD and click on the downward pointing arrow button.

3. Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the Utilities menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu. Select Restart from the Apple menu.

4. Reinstall the 10.10.4 update: OS X Yosemite 10.10.4 Combo Update.

5. Reinstall OS X: Reboot from the Recovery HD. Select Reinstall OS X from the Utilities menu, and click on the Continue button.

Reinstall the 10.10.4 update: OS X Yosemite 10.10.4 Combo Update, if required.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

I too found that the App Store would no t load or just crashes and even the combo update that I downloaded, hung right at the beginning. Now I use a very stock setup. I don't modify OS X in any way and pretty much use Apple stock apps for everything. Until today I never had any issues with software especially updates, but 10.10.4 has been nothing but problems. What finally worked for me was updating in safe mode where it went off without a hitch. I skipped the PRAM and SMC resets since they are related only to hardware issues and have no real effect on software. So I am just floored at how bad OS X has gotten with reliability.

Just adding another data point. I had the exact same problem (iMac (21.5-inch, Late 2009)). Starting in safe mode then restarting fixed the problem for me too. Also starting in safe mode was super slow and glitchy for me, I thought it had crashed at one point, but it did eventually let me log in and restart so be patient.

KEEP IT SIMPLE

Before starting to do all the complicated technical stuff which most answers suggest just try simply powering off:

a)Shut down

b) Unplug the power cable.

c) Wait at least one minute (make a cup of tea or something).

d) Reconnect power and start up.

e) Open the App Store and click the 'Restart' button next to the update.

f) Bingo. Go make another cup of tea and wait 20 minutes or so for the installation to complete etc.

This worked for me after 10.10.4 had failed to install twice previously. It also worked when other updates had failed.

Had the same issue. Was solved in using the combo update !