Incompatibility between Bfile Syscalls and Timers
Using Bfile syscalls while user timers are installed can cause SYSTEM ERRORs and other undefined behavior, especially with syscalls that change data on the file system. This is because the Bfile functions perform storage memory optimization whenever it is needed. Storage memory optimization - the same operation available from the Memory menu, except that here it's done in smaller steps - shuffles data around in the flash, meaning that the add-in code location often changes. This is usually not a problem, because once optimization finishes, the virtual address of the add-in ROM is remapped to the new add-in location and execution continues without issue. The problem lies on the time elapsed between the start of optimization and the updating of memory maps.
If there are timers running, they will keep running as optimization is performed. During the aforementioned period of time, the add-in ROM (and, thus, the handlers) will be mapped to something that isn't the add-in code anymore, meaning that if a timer handler is called, execution will jump to whatever is there at the time. This usually causes a hang, reboot or SYSTEM ERROR, but in the rare case that there is meaningful code at the address (for example, another add-in), it will get executed, possibly causing data corruption or even a full device brick (if the random code happens to erase the first flash sector, where the one and only bootloader, CASIOABS, is located).
To avoid these problems, always stop and uninstall any user timers before calling Bfile syscalls and, if necessary, don't forget to install and start the timers you uninstalled, when you're done with Bfile.
Putting timer handlers in a section of RAM (or even flash) that is not touched during the Bfile operations, may also be a valid approach to solving the problem, if timers must absolutely be kept running during the Bfile operations.