Where should I discuss SD/MMC FAT issues?

[JD123]

Somewhere up towards the top, you've got a sector loop that runs 0-512 bytes. should be 0-511???

That's an error. I'll fix it.

Figure out which byte variables you use the most (not pins assigned to a variable, but a RAM variable), and assign those to BANK0.
A variable assigned to BANK0 only needs a single instruction to set the PICs page register to ZERO. A variable assigned to other banks usually needs 2 instructions to set the PICs page register.
For instance, you've got:
w_byte var byte
try:
w_byte var byte BANK0

That's good to know - I had no idea. I'll give this a try. Thanks!

(I could really go nuts with the colons here!)

You mean in the text format? I've been known to make the text file shorter doing this, but for 'me' it's easier read the logic without hunting down colons.