Share your experience!
When you put a lot of data on your Xperia S (I've for example about 24Go used on my internal memory), PC Companion or Sony Bridge for Mac will fail to detect the phone because they will time out before the phone is ready.
The culprit is the MtpApp process on the phone which takes time to get the phone ready soon enough when there is a lot of data on the internal memory, and PC Companion or Sony Bridge for Mac don't seem to be patient enough 
The solution:
On PC with PC Companion:
On Mac with Sony Bridge for Mac
The following should only be done the first time:
This avoid Sony Bridge for Mac to start automaticlally and so too early before the phone is ready.
Once this done you just have to:
If the phone is detected as "Sony Ericsson Sony Ericsson Xperia™ NX" instead of "Sony Xperia™ S":
Hope this helps.
Message to Sony: You should really fix that connection issues, it's clearly not normal to have to do all what I described here to get the phone correctly detected by the PC or Mac.
Solved! Go to Solution.
Nice, thank you for this. I am sure many users will find this guide very helpful. 
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How about mounting the Xperia S to an iMac w/o sony bridge?
on leopard, snow leopard and lion
Mac OS X doesn't have support of MTP by default, so it won't mount automatically by default. But you can mount it using a software like SyncMate (http://mac.eltima.com/fr/sync-mac.html).
Also the bug I explain here also occurs on PC without PC Companion, because it's the MTP driver that time out, so even without PC Companion on PC you have to wait for MtpApp to finish and go to Device Manager to deactivate/active the Xperia S/MTP USB driver.
Also Sony Bridge for Mac and PC Companion (or Software Update Service) are required for software update and both are affected by this issue (time out before the phone is ready). So with or without PC Companion or Sony Bridge you'll have to do the trick I explain here to get access to your phone (except for SyncMate on Mac which seems to work fine even if you don't wait, it's because it uses its own software on the phone to communicate with it (i.e: you should also install the SyncMate app on the phone) and doesn't rely on MtpApp process).
Excellent!!! thanx for the info
Nice, thank you for this. I am sure many users will find this guide very helpful. 
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As some users still have issues, I'm updating the thread so it gets more visible
If anyone has updated their Xperia S to ICS, which now has native MTP support, I can suggest downloading the Android File Transfer app (http://www.android.com/filetransfer/) which almost instantly detects the internal storage and lets you transfer files.
Clearly the Sony Bridge for Mac is badly broken (it doesn't usually find the phone for many, many times - and can take ages) but Sony has said V3 is coming. When I do not know.
I'll try the fix above (thanks!) but think I can cope with the 'official' Android app for now as it's only really file transferring I need to do via the Mac most of the time - to transfer or backup files.
I also wholeheartedly recommend AirDroid (downloaded to the phone) as another way of managing files.
Of course formatting the internal memory does solve the problems, but it's not much a solution, as you'll have to backup, format internal memory, restore data afterward, which is eating much time and can be avoided with the solution I describe here.
Also since ICS update the Xperia S use the Android native implementation of MTP and the problem that showed on Gingerbread should not appear with ICS for PC Companion, the phone should be detected just fine even if the internal memory is almost full.
On Mac, the problem is totally different, since ICS update Sony Bridge for Mac doesn't detect the phone at all anymore whatever you do, so for now either you use PC Companion through a Windows virtual machine, or you use Android File Transfer app from Google, until Sony Bridge for Mac gets fixed.