RPi hangs when reading/writing to SD(FS)

I currently experience random hangs when trying to read from/write to the microSD in the Raspberry Pi B+. I am preparing a disc image for the CC2016 show, so did a lot of copying – the card is a Lexar 32GB, around 4GB is used. The card was initially prepared with SystemDisc and has a 64 MB DOS partition. The RISC OS nightly build from 2016-08-29 is used.

It worked OK for quite some time – after all, the 4 GB have been filled with various stuff.

If the same microSD card is used via USB, everything works as expected, no hangs.

The latest Pi firmware is used.

Any idea? It sounds a bit like an incompatibility between the microSD card and the RPi.