Missing internal flash memory and SD card does not mount

Some time ago I installed a custom firmware (YAR28 ) on my Archos 7HT. This worked fine for a while, until some day when I switched it on it showed the initial startup screen, and after that all my installed apps were gone. So I re-installed the stock firmware from Archos. But then I found out I could no longer access the sd-card. It was not mounted when I inserted it. But when I connected the device to my PC using USB it did show up as drive F: and G:. I could access F:, which was obviously my sd-card, but G: (which should be the internal flash) could not be accessed, and Windows even asked if this should be formatted. When I open the Files on my tablet, and select 'SD Card' it gives the 'SD card is unmounted' error. And when I select 'Internal Memory' it is completely empty. So it looks like the file system is still working, but the internal flash part somehow is no longer accessible. I have a feeling that some low level setting has changed and that I could somehow solve it if I could get access to the underlying Linux system, but I do not have a clue how to proceed.