Electronic – Use SDIO pheripheral for quad SPI flash memory

FATFS is a bad choice to use directly on a flash chip without any intermediate translation layer, as writing the FAT will overwrite the same sector again and again, causing these cells to wear out quickly.

1. You cannot reliably determine whether a certain block has been erased. If it reads back as only ones, the chance is fairly high that this is an empty, usable block, but it could also be a block that was in the process of being erased when the power went out, so some of the bits might be unstable. A flash file system should be aware of this, and explicitly mark blocks it has successfully erased with a specific marker.

2. The only way to be sure is to either perform an erase immediately before, or to have a note somewhere that a certain block has been erased.

3. You will erase the entire block, which contains 8 sectors. If any of these sectors already contains data, it will be erased as well, which you want to avoid.

As said, FAT is not suitable for use in this way.

1. You can use any type of flash memory that allows you to write at least as fast as the encoder produces data. That one allows ~84kB/s to be written to it (3ms worst case write time, max 256 bytes per page program operation), which should easily deal with the maximum throughput of the VS1053.
2. Most SPI flash doesn't deal well with PC-type file systems because the minimum erase size is usually several kB and the access speed is usually not great (e.g. 84kB/s write and the sector and block erase times are horrendous). There are minimalist embedded flash filesystems around that might do what you need, but they're so simple that it might be easier to just write your own rather than try to understand someone else's library.
3. There are cheap ARM MCU's with USB support and at least 2 SPI modules (1 each for the VS1053 and SPI Flash). Given the small volume of data, USB2 Full Speed (12Mbps) is probably adequate for your purposes so something like an STM32F042 might do the job. $3-4 in small quantities.