As you know, many of us musicians experience big trouble when connectiing audio interface to the late 2013 iMac.
I've been down my Apple store meeting one of the tech team who is also a musician, guess what : he told me he's still using Mountain Lion as he quickly found out an upgrade to Mavericks was not too reliable xhen it was issued en of 2013 !!!
Some say it has to do with the Fusion Drive unit (do you have one?), others say it's a hardware problem, the new HAaswell chip causing random I/O interruptions in the USB Bus, etc etc.
After buying a brand new 21" i5 iMac to replace the previous (also with FD, MAvericks) that was running fine, but got stolen unfortunately...
I've been stuck for 3 weeks now, in touch with the Focsurite tech-team who are willing to help (issued 2 new drives, 1.7 and 1.8b that fix some problems but leave the main trouble uchanged), ...
I'm quite desperate: it is simply impossible to record under these circumstances, you get a dirty, glitchy sound right there inside your track !!
It seems all we can do is hope and pray for this BIG **** won't be lasting too many weeks anymore.
Otherrwise : just get your money back! Personally I'll wait until, say 10th of March to take that decision...
Meanwhile I'm trying other soltuions : disconnect /switch of the FD device, run the sound trhough Soundflower, and see what happens, as a "solution provisoire", just to keep working ... as we all need to !
This is the recent answer I received yesterday from one of the Focusrite engineers, worth readiing :
Hi Jean
Sorry for your continued issues. I don't believe the issue is caused by USB power saving, but instead, the Fusion drive installed in your iMac.
We have seen a number of reports from people using various audio interfaces (from other manufacturers as well) who are reporting audio issues when using the Apple fusion drive. Luckily, Apple have acknowledge the bug and are releasing a fix in OSX 10.9.2 (currently in 7th beta).
Unfortunately, I cannot offer you an immediate resolution but we expect 10.9.2 to be publicly released shortly. Installing this update should resolve your glitchy audio.
Kind regards,
--
David Emery // Focusrite Technical Support
http://www.focusrite.com/answerbase/en/category.php?id=108
Focusrite Audio Engineering Limited
Windsor House, Turnpike Road, High Wycombe HP12 3FX, United Kingdom