Mounting android phone to android tablet I have a Samsung Galaxy Ace, with Android 2.3; and an android tablet with USB host support, running on 4.0.3.

When I connect the phone to the tablet with an OTG cable, tablet says SD card is broken, and that I need to format it.

The tablet is rooted and I have Debian on it, which is run alongside android (With the help of Sven Ola's 'Debian Kit').

Is there a way to mount phone FS on the tablet?
I think that 'broken' means 'unsupported file system'. You can try searching for solution even just in Play Store. I had similar situations what I tried to connect ntfs pendrive to smartphone. This helped me, but I feel like You do not have ntfs file system there. 

Hope that it somehow helps.