indexed-prefix Not Working as expected

[fraser125]

tl;dr; Adding 1 char to the length of the first part of the filename causes E34 and is unusable.
I was attempting to prefix the image files with "RolandMC50_" so I knew the source when viewing the image names. I found that RMC50_ worked and then I gradually increased the length until the steps were reproducible.

Additional Details may be available: GitHub Discussion

Working Example (Successfully formatted images with the Roland MC-50)

.\FF\FF.CFG

nav-mode = indexed  
indexed-prefix = "RoMC50_"

.\FF\IMG.CFG taken from Roland IMG.CFG in this repo

2 image Files in the root of the USB Flash Drive
RoMC50_0000.mc.img
RoMC50_0001.mc.img

Failing Example E34

Only change these things (Case Insensitive)
indexed-prefix = "RoLMC50_"`
Rename files to match
RoLMC50_0000.mc.img
RoLMC50_0001.mc.img

This makes the IMG.CFG line 1 character longer and the filenames 1 character longer. I believe one of these 2 are causing the E34 error.
I really lean towards the Filenames being to long because
The first block of the name goes from 12 -> 13 characters. ex. RoMC50_0001 (12) -> RoLMC50_0001 (13)
I tried to find the possible code areas for this issue, but C is not a strong language for me.

Obviously not a breaking bug and easy to work around, but nice to get it fixed.

[fraser125]

I'd like to confirm if my steps are reproducible. It's easy to work around this issue, but if it's not reproducible I could refine and verify the steps above.
Thanks!

[keirf]

Sorry, yes I understand this issue having thought about it. The documentation needs updating to make it clear that the prefix string is limited to seven characters, and silently truncates to that length.

[keirf]

On second thoughts, I haven't documented the truncation. I may in future change the behaviour to raise an error instead. But the seven-character limit is now documented, including in the example FF.CFG file (pending next release of FF, which might be a while!).