I had a problem when upgrading from 5.1.0.44 to 5.2.0.26.

I’d see the following:

Saved the running configuration to startup successfully

Then the CLI would just hang, after an hour I CTRL+C ‘d out of it.

Stopping the ACS process and re-running the install fixed the issue.

ACS/user# application stop acs

Stopping ACS.
Stopping Management and View............................................................
Stopping Runtime........................
Stopping Database..............
Cleanup....

ACS/user# application upgrade ACS_5.2.0.26.tar.gz repo
Do you want to save the current configuration ? (yes/no) [yes] ?
Generating configuration...
Saved the running configuration to startup successfully

     <Still hung for a very long time here>

% CARS Install application required post install reboot...

Broadcast message from root (pts/0) (Tue May 29 13:48:54 2016):

The system is going down for reboot NOW!






Application upgrade successful
ACS/user#

Running a show process from another terminal after starting the upgrade showed that the install was working fine this time. I could see processes like tar, gzip and rpm running.