Boot drive (SSD) going read-only randomly I've got a mid-2010 MBP running OS X 10.8.4. I've got a Sandisk SSD as my boot drive in my Super Bay and a 1.5TB hard disk in my HD bay. (I realize I could get better SSD performance if I swapped those, but the HD is too tall.)

When I boot my computer, everything seems fine. But after varying amounts of usage time, the SSD goes unwritable until I reboot. It's extremely weird. I can't touch to create new files anywhere (I get an error message like "touch: test.txt: Invalid argument"), I can't save anything, and applications like Mail, which depend on disk access start crashing without error messages. The Console and logs don't seem to be revealing anything unusual. Disk Utility doesn't start and diskutil hangs. If I manage to run out my main memory, I get the "Boot disk is full, kill an application" message. But even more weirdly, for apps that don't need to save anything, the computer seems to work fine. Rebooting restores everything to normal temporarily.

I'm not sure if it's related, but I also have these sporadic periods of 10-20 seconds of spinning rainbow ball, where kernel_task is my highest CPU user and seems to block ALL other activity. Disk transfers, network transfers, and all else seem to come to a temporary halt until whatever it is its doing finishes. I can usually still Cmd+Tab to Activity Monitor during these seizures.

I brought it to the Apple Store today, and their diagnostics couldn't find anything unsually. They successfully repaired permissions in Safe Mode, but the problem still exists. The tech there said that it seems as though the kernel can continue writing to disk in this state, but nothing in user mode.

Anybody have any idea whatsoever what might be causing this? My only other option will be to reinstall the OS, but I'm trying to make that my last resort.
It is possible that you are using a disk driver such as fuse. If so, uninstall it. 

I also have a 2010 MacBookPro with an SSD in my optical bay and a 750 gig drive. I found myself in a similar situation; unable to boot because my data drive randomly became read only at reboot.  

The culprit was fuse. It decided to remove my mount point, and create a symbolic link in its place. 

Try booting into safe mode by holding down the shift key, and removing any fuse like programs. 

Check /Volumes for any symbolic links (Except for the symbolic link to / )

If the issue persists, try creating a new user account. 

Lastly, because both sata ports are sata 2, it is unlikely you would see any performance increase by swapping drive locations.